The adapter is 100-240V, so it should work. Of course you'll need to adapt the U.S. plug to UK. The new Roku 2s always output 60 Hz in 1080p mode. I don't know enough about HDTVs to know if that's a problem or not.
The power adapter does work on 240v in the UK you just need to buy a US 2 pin to UK plug adaptor, which can be bought on ebay, amazon etc.
The HDMI hi-definition 720/1080 will work on all TVs in any country, it will also work with most wide screen computer monitors, providing you have the right cable.
Standard definition will work on most TVs made in the last 10 years as most of them are multi standard and therefore support NTSC.
The only thing you can't make to work right is the time zone clock settings on the box as there is no suitable time zone out side the states. It would be nice if this could be sorted out.