I don't believe that you will be able to keep the BT mail address.
What you could consider is registering your own domain - lots of advice on this forum! - with a provider, which will allow you to use any ISP email service to actually receive and host your mailbox(es), as it is done with a simple redirect to the actual mailbox(es).
The beauty is that your own domain email address won't change, but you can very easily change the back-end mail provider. Everyone you communicate with is only aware of your domain email address, and is unaware of the underlying mailbox address - and the fact that you can switch it at will.
Perhaps a bit late for your BT situation, but worth thinking about.
SD