Which religion, extant in the U K, do you know least about?
Same answer as Jeremy - some of the really small minority religions are so small that there are perhaps some that I've never even heard of. Off the top of my head, the ones that I don't know too much about are Zoroastrianism and Rastafari.
Which religion, extant in the U K, do you know most about?
Probably Christianity. Of the main religions I try to learn something about all of them, which is particularly easy to do not just through books but because of living in a place such as Leicester (which incidentally, Rhi, has the largest Jain temple outside of India. And very lovely it is too, well worth a visit) because it's such a melting-pot of religions and cultures.
Which religion, extant in the U K, do you wish you knew more about?
All of them. Religion is a fascinating phenomenon.
Which religion, extant in the U K, do you wish you knew nothing about?
None springs to mind. As Jeremy said deliberately wanting to know nothing about something (as opposed to being merely uninterested in it) is a worrisome phenomenon.
Which religion, extant in the U K , do you wish was not so, and why?
Like Rhi, radical Islamism, although to that I would add any formalised, organised, institutional form of religion. Private, personal belief is one thing; but if enough private believers club together, at some point they achieve a critical mass where they think that they're entitled to start demanding extra rights and special privileges, including the most special privilege of all - telling others (of a different religion or of no religion at all) what they can and can't watch or read, who they can and can't marry and so forth, generally how to live. We can do without that sort of carry on as of yesterday.