It is best NOT to have an email address linked to your ISP.
So set one up in Gmail for example (
[email protected]) which is independent of your ISP.
Then whatever ISP you go with you can keep your email address.
If you do set up a gmail address then tell the people who send you emails on btinternet one by one of your new email address.
However it can take many months to move everyone across so make sure you keep checking your btinternret address for many months afterwards.
I had an old email address I stopped using but I logged on every 6 months or so. About 4 years after I stopped using it I found an email from an old work colleague as this was the only email address he had of mine so it can sometimes take years to move everyone across to a new email address.