PlusNet is, of course, just BT under a different name anyway.
As has been written many times here on AB though, you simply shouldn't be using an email address provided by
any ISP. It's one of the ways that they try to tie you to their services. Instead, use an independent provider.
I recommend GMX:
https://www.gmx.co.uk/
If Losgigs changes her ISP to PlusNet, I suggest immediately setting up a new email account at GMX and then using the 'mail collector' facility to get mail from her old BT address (which will remain active for 90 days without payment) automatically forwarded to the new GMX one. That way, Losgigs won't lose any mail and she'll have three months to ensure that people know her new email address.