List of HD movies on Netflix (via TiVo HD or xbox 360) http://www.isitinhd.com/
List of HD movies on Apple TV (not sure how many are available to UK)
Apparently the only TV show in HD for UK Apple TV is Lost season 5. £2.49 each episode or about £42 for whole series
New AppleTV HD releases
Here's the xbox 360 list of SD and HD movie downloads
The UK xbox 360 has 150 HD movies as of March 2009.
Anyone know how many a UK Apple TV has?
March 2009: You can now BUY instead of rent HD movies from Apple