Exactly as a couple of previous posters have noted, don't look at the PQ of a screen that's on display at Tesco/Comets etc and believe that that is the image you will get at home!
I've seen full 1080 screens running the BBC's HD preview channel that looked awful! The fact that the TV's had been taken out of their boxes and plugged straight in, and had not been correctly set-up is hardly a way of demonstrating the TV's full potential and gaining maximum sales.
Read the magazines before you make a decision to outlay you're hundreds of pounds! A good review is a good good review and should be used as a pointer for a demonstration.
Are you just going to use this TV for watching TV and DVD's, or for gaming as well?
If you are going to use it for gaming then I would suggest LCD over plasma.
As much as I've read about plasma screen burn not being an issue any more I 've seen my mates brand new plasma do just that recently when leaving a game on pause for a few minutes!
As already noted I would look first at John Lewis as they have great deals on quality LCD's & Plasmas with 3-5 years warranty thrown in free!
Avoid taking out credit on a set at Currys/Comet etc, as they can charge around 25% interest!
I saw a 42" LED TV on Sunday at a shop on Tottenham Court Road. It was going for £1700, but the PQ was astonishing!
But I know that in a years time their will be better LED TV's around at better prices.
Damn that's my next big purchase next year!