Hi RobertK,
I am thinking the general perception of poly in the UK is probably similar to what it is in the US. To begin with, most people have not even heard of poly, and most have not considered it one way or another. Of those who have considered it, most look upon it with wrath. It is considered to be an unhealthy, immoral, destructive way to live and love. There are, however, some open-minded people that are at least willing to say, "Well, it's fine for other people, it's just not for me." Now as for percentages I have no idea. I just have a very broad notion of "most, some," and what have you. And that is only based on my personal observation and experience.
Hope that helps,
Kevin T.