I live in Korea and own a Japanese Wii, to answer some of your questions :
Regarding VC games : With a jap Wii you can buy japanese VC games from anywhere in the world and can even pay by credit card from your home. Only problem is the menus are in japanese, but the catalogue is bigger.
1. It should work in the UK if you have a modern TV : most modern TVs can read PAL/SECAM/NTSCUS and NTSC/J (japanese norm) however you should check your TV manual to make sure it can display NTSC signal (some TVs don't, especially old ones)
2. You can only play NTSC/J games with a japanese wii. At the moment that means japanese games in japanese only. However, with the launch of asian version wiis for Chinese new year (Mid feb 07), asian version games (compatible with japanese wii) will be released. Most of these games will feature english language.
There are alerady quite a few asian version gamecube games that are in english and work with the japanese wii.
3. Conversion kit will be needed only to play european/US games. They do not exist yet.
4. The biggest differences will be :
- You will be able to get the japanese/asian exclusive releases
- You will have to import all your games (until a freeloader comes)
- The Wii menu, weather and news channels will be in japanese only
- Probably, online play will only be available with JP games owners (which is why I made a thread for JP wii owners to exchange wii codes)
Hope that helps ! Don't hesitate to ask if you have more questions.