Whether the Apple TV is worth it in the UK totally depends on what you want.
If you want to rent films and TV shows then there may be better options.
If you only have one TV that you want to watch it on, and your computer is near that TV then you may want to consider just plugging the computer into the TV instead.
If you have a library in a variety of formats, and hate the idea of keeping a computer on all the time then there are almost certainly better options.
However, if your library is in iTunes and you are looking for a way of streaming to one or more TV, and are happy to have a computer on then it is surely the best option.
I have an iMac which holds my library and is always on. It also has an EyeTV card to record any TV which, as mentioned above, is easy to edit and add to my library.
I have two Apple TV's and the benefits are huge. At £99 (or £101 now) it is one of the best bargains that I have ever purchased.
However, I don't rent much (2 films in the 3 months I've owned it). As I said, it depends on your usage.
One final point. Lots of people don't like the idea of converting video for the Apple TV. If your library is mainly downloaded then you don't get much benefit, but if you have your own DVD conversions then, depending when they were done, you may get other benefits.
I first ripped my DVds as Avi's about 5 years ago. I decided to go for the Apple TV and re-rip. I've still not quite finished, but it's been well worth it. My old rips didn't have the surround track, and were terrible quality in comparison with my new ones. They are so much better to watch. Of course, if you only ripped a short while ago then there won't be much benefit, but I suspect a few people could do with updating their rips.