There was recently an article on the BBC about the extortionate charges ISPs sometimes levy if you move away from them but want to keep your email address.
BT could charge £7.50/month, TalkTalk could charge £5.00/month and Virgin simply delete your email account 90 days after you leave, which means you'd lose all your email.
It is therefore wise not to use your ISP's email addresses at all. You are better off using an independent email account that will survive ISP changes undisturbed. Pick a reputable, independent supplier such as Gmail, Outlook, Office 365 or Zoho. If you just want one email address, many of the independent services will provide that for free. You may have to pay for multiple addresses, email groups or custom domains.
I would even caution against using your own server (maybe via something like cPanel) for email. Like ISPs, this is something that might change if you move to a different server or web host. Sure, most control panels provide some way to transfer email across but it's much easier if you just keep your email separate.
I have my own domain on a server I control, but all my email goes through Office 365 Exchange. Emails still go to my domain name, so I have addresses @gordonansell.com rather than @outlook.com or whatever, and I just direct it appropriately via the DNS settings for the domain.
Doing this does of course mean you're beholden to the big silos for email and, as a fan of the IndieWeb, this goes against the grain, but I'd choose a silo in this instance because it's just so much more convenient.