With the vast majority of ISPs, switching to a different ISP means that you'll lose your email address altogether. BT are actually being rather generous to their former customers by allowing them to retain their old address for a very small monthly payment.
The standard advice (given in the technical press and here on AB) is that you should never use an email address provided by your ISP. It's far better to use an email account provided by an independent service (such as Gmail or GMX). Further, you should use different email addresses for different purposes, keeping your prime email address just for your friends and/or business needs. So, for example, the address you use to sign up to websites such as AB should be different to the one you give to your friends. (Then if AB's server was to be hacked by spammers, you wouldn't start getting thousands of spam emails in your main account). Similarly, you should use different email addresses for online shopping, etc.
I've got seven active email accounts at the moment but, over the years, I've used well over a hundred different email addresses.