I would go for a Sony Bravia if you can find one for that money mate, absolutely fantastic picture and colours.
Samsung are quite good though otherwise for LCDs.
The make does play a big difference though, you could buy a 50" TV made by some random brand, but I would rather go for a decent brand as you do get what you pay for in this technology.
For £800 you should be able to get the old Sony 40" LCD which we have in our lounge which is fantastic, plenty of ports in the back, great picture and clarity and the ubiquitous myriad of Sony option menu so you can change pretty much anything!
I would say if you have Sky or Virgin and you haven't already then an HD box is essential to make the most out of it especially going forward. The older NTL boxes Virgin still use made the picture look really washed out even on non HD channels before we upgraded.