An excellent way to make use of a Senior Railcard! I considered a similar concept with a 7 day FC ALR for last year, using one to both score off a ton of trains but also to see some of the best of Britain. I never did do it, but it's on my list of things to look at one day.
Anyway, input into this plan of yours. There's little chance that one plan is going to tick enough boxes in terms of satisfaction. What I would aim at doing is a mix of all 3 of your main plans there, so revisit some places you haven't seen in a mighty long time, do some of the track coverage while at it and do some of the infill bits on deviations.
What I mean by that is let's say you're on the way from the South West to Scotland. You'll probably pass fairly close or through Crewe, from which it isn't all that far to Chester, from where you could easily do a train to Runcorn or Liverpool over the Halton Curve. It isn't that far from Liverpool to Preston to get back on track to your primary destination, but it fills in a gap at the cost of however long the diversion via Chester would take.
Similar for that bit at Sleaford, which is actually a bit more awkward to do as a quick diversion off the mainline. Not impossible, of course, but taking a couple of hours out of a plan with a change at Grantham onto some EMR trains wouldn't be the end of the world.
Indeed, I noted you want the track from Worting Junction to Exmouth Junction, and are considering a nostalgic trip to Penzance. Depending on where you are, a ride on a 159 from Waterloo to Exeter St Davids would soon sort out the track need, and I recall First Class on 159s isn't too bad. A quick change onto a GWR IET down to Penzance, and might I suggest considering taking a bit of time out to also visit Land's End while down there? I've not been myself yet, firmly on the list to do, but would certainly qualify for a 'best of Britain' plan. Such an adventure would be quite a long day, so worth considering either the Night Riviera back to London or an overnight stay in Penzance, to allow more time to explore and so on.
There are so many beautiful parts of the UK to see, it's impossible to nail down more than a handful of suggestions. It does of course depend on your interests, especially if we try to restrict it to 'best of British railways'. For the purposes of this post, I'm going to make recommendations that include the best of the journeys themselves as well as the destinations, as much as I can. Some of them don't make full use of the First Class rover, I'll grant you that, but if nothing else perhaps this list will inspire you to potentially come up with even better plans. In no order whatsoever, I present a handful of some of the best the UK railway can get to/enjoy as part of the journey:
1) Transpennine Class 68s, others would have to gen you up on current workings but last I checked they operate to Scarborough. A First Class ticket puts you in the coach next to the loco, a class which is immensely popular. Scarborough itself, in my opinion, is a nice place and worth exploring, certainly it's on my list to go back to.
2) Bournemouth seafront, the 444s from London to Weymouth via Southampton and Bournemouth are nice enough trains but for this one I'm more focused on the destination. I also like Weymouth, I have to confess, but if the seaside is your idea of a holiday then Bournemouth and its seafront is crying out for you. The journey to Bournemouth also has railway interest with the Freightliner yards around Southampton, as well as Eastleigh and its yards and works. The journey has other interest, as it goes through the New Forest.
3) I know it has no First Class, but I really like the journey from Shrewsbury to Borth and Aberystwyth through the Cambrian Mountains. On a sunny day, it's really nice, and the journey north of Dovey Junction is also highly worth doing. I haven't done much of the journey north of the junction for a very long time, it is firmly on my to-do list and ideally so before the 158s get replaced (I have time, the 197s are still some way off yet), but the bit I did do a few years ago was beautiful.
4) Sticking with the Wales theme, and absolutely no First Class on this line either, if you want to experience one of Britain's more remote lines but also through nice scenery with the quirks of token operation on the lengthy single-track line, then I invite you to journey along the Heart of Wales line between Llanelli and Craven Arms. It does take a few hours, but it's one of Britain's scenic railways and worth doing if you haven't been before. I haven't been on it for quite some time myself, I could do with returning to the line and doing some more exploring myself actually.
5) Continuing with a 'best of Britain's railways' theme, there's nothing quite like enjoying the beauty of the scenery through the likes of Tebay, sitting in a comfortable seat and enjoying a tea/coffee, staring out of the window at the scenery. A Pendolino First Class seat is nice and comfortable, indeed even Standard Class seats are good in my view.
6) The other person who posted before I finished this reminded me that experiencing the Caledonian Sleeper with its Mark 5 coaches is a must. Class 92 haulage on the electric bit and Class 73 haulage is available beyond. The Fort William sleeper also has a Class 66 on the front. I found the beds to be pretty decent, and the bedding they use is truly wonderful. I haven't been on it since October 2020, it's just how it is for my journeys, so I don't know what it's like now but it is worth experiencing. I seriously want one of those pillows and quilts of my own, they're that good!
7) It wouldn't be right if I didn't recommend having a journey through the Cotswolds, there is some beautiful scenery to be seen and from the comfort of an IET First Class seat, with complimentary refreshments, oh yes. I know the seats aren't as nice as those that were in First Class on the HSTs, those were truly amazing, but they do the job well in my opinion. Multiple options for a plan, I do like the route from Swindon to Cheltenham (ignore Gloucester, it doesn't exist
) through the Golden Valley I must say. I do also like the route from Worcester to Oxford, and I am biased I know, with my location, but the Malvern Hills are not that far from Worcester and worth seeing. Great Malvern is undergoing a canopy restoration at the moment, when that's finished it'll look amazing. Malvern Link, just up the road, has been done up in a GWR-like fashion and has its little quirky charms to it. Depending on how fit and healthy you are, Malvern itself is worth exploring and the hills most definitely are. They are an adventure of their own though, but if you're passing through then they are a beautiful sight. Take a seat on the right hand side in the direction of travel and gaze upon the beauty!
8) You can hardly talk about the best of British railways without looking at the journey between Manchester and Leeds, some of the most beautiful scenery England has to offer. The Calder Valley has plenty of beauty all around the place to enjoy from the train, or indeed exploring by foot. No First Class on the Calder Valley (Manchester Victoria-Leeds via Hebden Bridge) but a beautiful journey and strongly recommended. I touched upon some of it last month when I popped up to Manchester, and I absolutely need to go back soon.
9) We definitely, 100%, cannot while on the subject of gorgeous journeys not mention the Kyle of Lochalsh line. Beautiful scenery all around, and the journey goes through some of the most remote and least used stations in the land. The journey on the 158 will be well worth your effort, and yes it is on my list to go and revisit too, I haven't been on it since 2006. The line to Thurso and Wick is also nice, no question of it, but the Kyle line is better, much better.
10) Sticking with Scotland, I have to confess I'm not too clued up with where they run now but ScotRail have HSTs on a number of services, so you get the opportunity to travel in comfort. Someone else would have to confirm their routes, but worth going out of your way for if in the area. Perhaps combine it with a journey over what is probably Britain's most well known railway bridge, the Forth Bridge.
11) Last but not least, give consideration to a journey via the Cumbrian Coast line. Again, no First Class but it is worth doing. Back when there were 37s on selected trains, it was much more fun I will grant you that, but if you want a best of British experience then I would recommend it. It's not quite as high up the scale as the Cambrian Coast, but it is still worth looking at doing.
Of course, many if not all of these suggestions may not line up with what you want to do on your rover. I merely offer you some ideas, primarily based on experience, and there are no doubt plenty of things I haven't mentioned. For example of one such delight, I offer things such as racing along the Great Western Main Line at 125mph, in the comfort of a First Class seat, sipping coffee and eating biscuits, while heading to places like Bristol and Cardiff at high speed. I also neglected to discuss dining services such as the Pullman, partly because of a lack of recent experience of what they offer these days. I'm sure however you have got a taste of the things I've looked at, after all what I count as the best of Britain may be nothing like what someone counts it as.
I hope this post gives you some food for thought, and of course I am happy to expand upon any of my thoughts if required. The most important thing to say now though is simply this:
Whatever you choose to do, have an absolutely amazing time!
