Firstly you should be allocated a base before you start training. They used to keep you waiting until 1 month before your start date.
Take the base allocation with a pinch of salt though. I was told two months before the Refresher Course that I would be based at LTN. When I sat down on the first day of said Refresher Course, I opened my starter pack and it said "Welcome to Belfast"
Only took a few months to get transfered to LTN though, so no complains. I could even use the jumpseat to get back home when the flight was full. The hiring requirements seem to be so dynamic that I don't think they can really commit to a base until you actually start.
P