in the end I chose to remain with Orange / EE to retain my email address and see how their fibre service performed.
Using an ISP hosted email system is not, generally, a good idea if it ties you into the ISP as your has.
Now you have decided not to move, at least partly due to retaining your email address, it is clearly time to do something about it.
Either use a third part mail service (gmail, yahoo etc) or get yourself a domain and sign up with a hosting service for an email package (this is the prefered option). This gives you a unlimited number of email addresses ("[email protected]") and you can shift domain to any hosting company - you are not tied in!
I did this years ago and have not used an ISP mail system for 10 years or more. You only need to do it once!
I can't argue against the principle of having your own domain name from which you can create your own email address that you can move freely from one ISP to another; sadly it is not a feature used by many domestic users. An experienced Internet user will say it is a breeze to setup, so what's stopping wide adoption? I would say it is nothing more than the ISPs facilities being trouble free to setup and robust in use.
There are certainly a lot of hoops to jump through to have your own email address, the most obvious being actually finding a domain name that is not in use, and suitable for you. Then you have to make the fundamental choice of using forwarding into the ISP email system or having it hosted by an agency. Neither method is the gold standard. The two major issues are "reputation" with hosted services and the other is roaming SMTP availability. If 'greenglide' has not experienced either, he has been very, very, lucky.
An added complication is that with BT you have to prove that you are entitled to use your domain name, which delays you configuring your email client. Furthermore you have to definitely discover the names of the mail servers, their port numbers and security requirements for each server. There is little, to no, published information on this issue and that which can be found generally does *NOT* apply for use with your own domain name.
If you decide to have hosted email facilities, then setting up your email client is easy because your hosting company will provide all the information needed, but by definition you will be sharing a [computer] host with a common IP address. That IP address is unlikely to have a good reputation, meaning that your outgoing emails may fail to reach an intended recipient [especially if AOL] or be shunted into their Junk Mail box.
Using an ISPs SMTP server won't have those problems but you *may* be restricted to outgoing facilities only being possible when connect to your home network. A real pain if you want to send an email when out and about with your laptop.
All the problems highlighted above are resolvable but your average user will need a knowledgeable friend to help out or a deep pocket to call the ISPs help desk. It is very true you only need to go through all this performance once with ongoing benefits and flexibility gained, but it is a giant step for many to take in the first place.
David