We have a subdivision that provides IT support and I would suggest that no matter how much you dislike F2S/ TT Group, you would be better managing the migrations in a phased manner. A mass migration will move all the lines at the same time, so you will have a batch of routers to reconfigure and likely staff needed on site to ensure this happens in a way that doesn't cause upset to your customers and that is before anything goes wrong.
If I was planning this, I would pick off sites that are close together and put those migrations in at the same time, that way one or two engineers can be sent to that area to make the changes and get customers working with out too much down time (Down time because some times BT will reset the session as migration completes which will leave you with the wrong user name/ password details in the router. Other times the ISP that is losing the connection will hoof you off their LNS as you migrate achieving the same thing).
We can offer this type of project planning if that helps but if does mean that you will need MAC codes to be able to progress this. OfCOM might be a last resort if you can not get any sense over the phone/ email.