It's probably about 10 years since I did some research on them.Your winters are actually quite mild from my perspective so they should be able to work year around with a high COP. You don't get down to a COP of 1 until you get to -20C with heat pumps here made in the last 10 years (at least in the nordic market, I know the US market has lagged behind for a long time for instance). Heat pumps are a very popular heating solution in the nordic countries (the most popular perhaps) so I can't think they would have any problems in the UK winters.
I noticed recently that HM Government are pushing them so automatically assumed they're useless.
If I get my net connection fixed soon I'll have another look.