Ah, that explains the announcements I heard when I had the misfortune to have to make quite a few long-distance journeys on cross-country "please place you luggage in the overhead racks or under your seat" - I remember thinking at the time that this wasn't great as putting luggage under your own seat encroached into legroom of the person behind, but can see why you wouldn't always have a seat in front.
I guess that is an issue, but on FGW I think they have those kick down foot rests like many coaches have? Or did I just imagine that?
If there's some form of barrier under the seat to stop a bag being pushed right back, you could have a carry-on bag under your seat and perhaps sticking out a bit in front (but that's in your space and your feet would be further forward anyway). It also means your bag is easier to get to.
There's something to be said for people with large bags to consider having, say, two smaller bags that can be stacked on top of each other securely when dragged.. then you could put one in the overhead luggage rack, one under the seat, or possibly even both overhead.
When negotiating stairs (we still have a long way to go before the rail network is step free) you can separate the bags to carry them easier than a large bag.
I speak as someone who just came back from Sweden with a huge bag (weighing just over 25Kg and carrying an orange 'heavy' tag!) and wish I'd taken my own advice given above!