It's not just you, Barmaid. I've just tried to get to the login page (even though I don't use BT Internet myself) on with Slimjet, Chrome and Edge (all of which are based upon the same 'developer level' browser) and with Firefox (which uses entirely separate architecture). I've been seeing the same message as you in every case.
It seems that BT's website has an invalid security certificate. Such an error usually occurs with 'small scale' websites, where the owner has forgotten to pay the subscription to renew their certificate. It shouldn't happen with the 'big boys', such as BT.
However, whatever the reason, the BT site is currently trying to use an invalid security certificate (which proves that the 'https' secure connection is genuine). So LOADS of people are experiencing problems with the site at the moment:
https://downdetector.co.uk/status/bt-british-telecom/
If you urgently need to access your email, try using an app instead of web-based access to your account. (On a PC, the easiest app to use is probably the Windows 10 Mail app, or its Windows 11 equivalent, simply because it's already installed). On a mobile device, the default mail app (whatever that might be) should suffice.
If you're trying to access the BT website for any other reason though, you'll just have to wait until the problem has been fixed at their end.