Hello, I joined BAE as an apprentice in the early 2010s on the aircraft maintenance/technician side of things.
In my case, it was about a year of classroom and practical training in workshops and on some old Hawk T1 aircraft.
The second year, I moved onto an RAF base where you effectively train on the job working with both the RAF and BAE personnel. Back when I started, you were sent to Coningsby, Marham or Cottesmore and often weren't given too much say in that decision.
As I understand it today, the training centre is at Humberside Airport and the main locations are Coningsby (Typhoon) and Marham (F-35). Worth mentioning, the Marham location will come with extra security screening on top of what you already do for obvious reasons.
The pay is quite reasonable for an apprenticeship, better than most and often opens up a raft of career opportunities outside of BAE as well as internally as many have taken up. You can make a lot of money in Saudi Arabia for example but that's not for everyone's taste.
The one thing I would stress, is that at least on the maintenance side, full time employee pay overall is still a ways behind typical industry "standard". Staff turnover is also quite high these days I'm told from those who still work there.
I left personally as at the time, the salary wasn't close to being competitive. I'm told it's improving but has a ways to go. Not to say the apprenticeship can't give you a leg up overall. There is no service retention clause in your contract, so you could leave the day after you got your qualification for example. At least there wasn't when I was going through the process.
All of the above pertains only to the aircraft maintenance apprenticeship. I can't speak on the apprenticeships offered at Salmesbury/Warton.