Thanks for your reply.
The usage limit is the key sticking point really as I manage a charity's website and email comms and do a lot of uploading of event photos to Flickr and downloading website backups etc so I think I'll easily exceed the cap.
The bolt-on certainly sounds like a deal changer - thanks for letting me know about this.
RE BT offering FTTP, I have been trying to get them to offer fibre of any kind for 18 months and gotten nowhere. Their call centre staff haven't got a [censored] clue, not blaming them but obviously the info they're being fed is useless.