The main difference apart from routes and traction, is rostering.
Thameslink depots (that's those on the south side of the river, and also those on the Midland Main Line) have the following rest day pattern:
Sun, Mon, Tues, Late, Late, Late, Late,
{Late}, Late, Late, Wed, Thurs, Early, Early,
{Early}, Early, Early, Early, Early, Fri, Sat
Sundays above are in {brackets} and some throughout the roster will be worked on the shift shown above, whilst others will be a day off. Usually about 50% of Sundays are rostered to work throughout the roster. You have to commit to 12 per year, but you can offload your commitment to someone else if they want to and are able to take any off your hands.
This shift pattern gives less changeovers between types of shift, and gives two rest days between lates and earlies, which is easier on the body clock than other shift systems. A downside is that it's next to impossible to permanently swap for a certain shift (AMs or PMs), as one or other of you is intertwined with either the next week or previous week. I think nights is usually instead of a Late week.
You can't really swap rest days as you have the same problem of certain rest days being tied into certain shifts.
It gives a long weekend every three weeks, and you work two out of three Saturdays. Because it's the same rest day pattern repeated you'll know what your rest days are in your birthday/Christmas week in in 2022, 2023, 2024..... This is quite good for planning ahead.
=====
The GN has a different rest day and shift pattern:
Sun Mon Tues,
{Sun} Wed, Thurs,
{Sun} Random weekday + Sat
{Sun} Fri, Sat
This is the general pattern at most GN depots (apart from Kings Lynn I think).
Shifts are AM one week and PM the next, but every so often there'll be a week of nights after which you go back to PMs the following week. (Very occasionally I have seen a roster where you might go back AMs but you'd have a long weekend between the nights and AMs).
You get a long weekend off every 4 weeks, but a lot more Saturdays off with roughly 50% worked (instead of 66.6% at TL depots).
There are about 50% of Sundays worked on the GN, with again a commitment (which you can offload) to work 12 per year. They don't pop up in the middle of a long weekend, but can appear anywhere else.
Unlike TL depots the GN rest day pattern is not fixed in stone though. It can sometimes change at a timetable change (May and December) with the December one being a particular pain as Christmas is fast approaching by the time the new roster is posted in late November.
Because the shifts change at the weekend, it can sometimes mean Lates Saturday, Sunday off, Earlies Monday, which isn't so nice on the body clock.
However, it does mean you can swap weeks with people easily. In fact quite a lot swap permanently with someone else, doing a week of their own earlies followed by a week of their swap partner's earlies and so on. Or vice versa. You'd just do your own nights when they came round - although... there's often quite a few who want to swap to those.
Swapping a rest day is easy too. If you want a Tuesday off and you are PMs just find someone else on PMs with a Tuesday rest day and see if they'll swap it for one of yours that week.
This is shift system in place on the GN at the moment. Some time ago the company did express a desire to standardise it to the Thameslink system, but as there are a significant number of drivers on the GN who permanently swap shifts, as well as other issues, nothing has been heard about it recently.
Some Welwyn drivers will route learn to Sevenoaks soon, at which point I expect the depot will be split into two links (rotas).
They currently sign 717s and 700s and have a mix of inner and outer suburban.
You may find, as alluded to another thread thread that you actually end up being offered Hornsey inner link on 717s as there is a small number on the transfer list waiting to go to Welwyn at present.