What's the advantage of leaving the phone part on the BT network?
BT phone service has never been a problem so I see no need to change, other than via BT wholesale. If I go fully unbundled and it turns out to be a bad move, I then have the problems of getting back a BT line back and so on. There's only TT and Orange LLU on the exchange.
OK the line is not very good for broadband, but I doubt whether moving to a new line would be any better since none of my neighbours are getting near the speeds I get; in fact some are getting little more than dial up. The only *possible* advantage in a BB move might be the different DLM.
Things are not yet quite bad enough to move, although that could change if the line deteriorates further. BT have said recently that the area will get FTTC "eventually".