Use your PC. Your TV should have either a VGA or DVI input, so you might need a converter (couple of quid on ebay).
Download some HD demos and check that they play alright on your PC first, if 1080 stuff is juttery, try 720p. If you don't have broadband, buy a couple of DVDs off ebay that someone else has filled with their downloads. Only MAKE SURE you DON'T pay more than a pound or two at most, as anyone charging more than cost for burnt DVDs is really taking the ****!
Better yet, download the BBC planet earth series using bittorrent if you have broadband. I don't think this is dodgy because, as a TV licence payer,
you've already paid for it, and the BBC air their HD stuff free for anyone who has the equipment anyway.
Just been watching it on my new set. ABSOLUTELY BREATHTAKING!
Another fantastic free HD clip is
Elephant's Dream - An open source CGi short movie. This looks incredible, rendered in native 1080p, it is about the best example of HD available anywhere. ...Just don't ask what the plot is about!!
P.S. Mediaplayer Classic is the best player to use... search for it on sourceforge.net