BT only use web forms so that each message is given a tracking number so its progress can be followed within BT.
This is the starting point.
Hello, I've tried that link and cannot seem to get to the web form (I also tried a different URL which used to work some while ago) - do they keep hiding it ??? You can "Complain about moving home" (?), presumably via chat or calling them, but the email form remains elusive.
Eventually I found it, at URL https://bt.custhelp.com/app/contact_email/c/1885 - this form proved frustratingly difficult to filll out, as in the middle of logging a complaint it would clear itself and one would have to start again (I suspected it was a "deliberate" feature) - I first would write the complaint details in word pad and then pasting it until I could submit the form.