Funny, but in my personal experience, most Pagans are lapsed Christians and the most common reason given is that it, Chritianity, is discredited in its origins and its dognmatic refusal to see the errors in its history as taken from the Bible.
I don't doubt that, but I'm not sure that is inconsistent with my comment.
Research suggests that religious people as adults tend to have been brought up religious. In most cases they've remained in the same religion, but a few jump to a different religion (not too far from from the religion they were brought up in), so it doesn't surprise me that your experience is that most pagans are lapsed christians rather than former non-religious or atheist etc.
Also I suspect few people in the UK are brought up as pagan, while many are brought up as christian so again unsurprising that lapsed christians are an active pool of potential pagans.
Finally christianity in the UK (and northern europe certainly) has always included a major dollop of pagan elements in many of its festivals and activities, from christmas and easter, through harvest festivals etc. So I can see how paganism may be very attractive to ex-christians, with plenty of familiar elements, but the non-sense stuff taken out.