Buy/Download HD Movies?

I want to get into some good films, partly in order to help brush up my knowledge on popular films. I do a lot of quiz nights, and movie questions come up very often and haven't been able to answer any correctly!

Can anyone recommend some good UK sites where you can buy to download movies? Preferably in High Definition and in 5.1 surround if available would be a bonus.

Just to note, I'm looking to buy films outright from legitimate companies not rent them or pirate them. Renting could be an option but I might need to go back and cross reference something, in which case it works out more expensive.

Also can you recommend any good films that you've seen or Blu-ray/DVD/Digital or in a cinema? I'm mainly more into Action and Comedy but I'm willing to broaden my horizons within reason :)

Thanks!