Whilst some are territory related, there are a number of interesting ones relating to specific TV series. I’ve never been good an unearthing this kind of back matter, although I’m always intrigued when I do find it. My current favourites are http://www.unit.org.uk, which has some interesting UNIT activity relating to the Chris Ecclestone era Doctor Who, and http://jellyparties.co.uk/, which is the truly terrifying website of Psychoville’s Mr. Jelly.
Most of the sites don’t look like they have taken a lot of time to set up, although I suspect the awful state of Mr. Jelly’s is the work of many hours for an ingenious designer. Perhaps I should make an FOI request myself as to how much time these sites take, and how many other people have found them before now?