Firstly, a massive well done to all those that made the cut, and hard luck to those that didn't.
Davey, start dates were allocated last year, and there wasn't much leeway for change. I made sure to let BA/APL know that I was available to start immediately, and would have preferred a date as soon as possible. I wasn't the only one to do this, and ultimately when the dates were issued I was given a date in the middle of the range. I suspect the only way for things to be changed would be to have good legitimate reason; and to find someone in a similar position willing to swap. Even then...
I think everything took a little longer than planned during our early admin stages so there was a bit of a delay between receiving the email you've just received, and getting a start date. To then discover it wold be another 6 months before training began was slightly disappointing at the time, but the gap flew by, and many of the administrative creases had been ironed out in time for us to complete the required (and not insignificant) paperwork. I would also say that, whilst there is a lot of admin to sort, you will be given timely guidance regarding its completion. Those that need to know soon will. Those with later dates that may be yet to be decided will find out in due course, but in plenty of time. The one thing you can do is get an Initial EASA class 1 medical which must be done at Gatwick, and of course, it'll cost you. Those who already have medicals but may require renewals may be better to wait until BA get in touch - they may ( I stress 'may') be able to save you some cash.
All the above is of course my observation only, and any issues/queries you should have may well be better addresses to the BA team directly. I look forward to meeting some of you folks when the time comes, and when I'm not spending my evenings immersed in the intricacies of aerodrome approach lighting specifications, and instrument approach design. That's what you've got to look forward to!