The "usual" situation is that when you change ISPs you loose access to your email address.
BTs official line seems to be that that is true and if you want to just maintain access to your email account and not use BT as an ISP then you need to pay some small charge per month.
However some people are saying that they can still access their email after they leave BT, some of them saying that they've been switched to use some sort of dial up account every few months in order to keep the account live.
I have found some ISPs that I used in the past would keep email access up for quite some time after the actual death of my account, however one day they would then just spontaneously die.
So unless BT tell you for sure that you'll be able to keep access after you leave them then I'd consider any access that you do get to be something that could be taken away at any moment.
My advice? Set up a free email account with someone (for example Yahoo directly or Gmail), start to switch over and count any time with your old email address after you leave BT as a blessing rather than something you can rely on.
The best thing about free email accounts is that they give you the freedom to move ISP as much as you like, oh and that they don't cost anything