When I joined BT they gave me an email address so I started using that and gave up the previous address with ANother. You can of course have other emails with other providers if you want to.
Now you have to change you'd be better off setting up and using an email address not connected to your ISP so if you change ISP again in the future you don't have to change email again.
I find the BT emails to be good, and they are very willing to come on and sort out any problems with their remote access where you watch as they look into your settings.
I'm same as you Hopkirk and have been looking at switching to BT who say I can keep the .tiscali address. However I understand that it's Tiscali/Talktalk that won't let us keep it.
During setting up the new ISP, you'll have to accept one of the new ISP's email addresses, but as at that time you only have the new ISP addresses available, then your old email address is unreachable, until you finish setting up the new ISSP email address. Hence you'll find your Tiscali email address will stop working through your new ISP.
BT (and any other ISP) will let you receive mails from any email provider, talk talk/tiscali won't supply you with an email address through them if you're not paying them anything.