I know LCD technology is improving all the time, but at that size - i.e. 46" and above, Plasma still rules the roost for picture quality. Better blacks, less noise/digital artifacts and, to a lesser extent, better colours.
As has been said Pioneer and Panasonic are 2 of the best - even for LCD screens and not just £3k plasmas.
A HUGE consideration with that size of LCD is how close will you be sitting to it? The closer you are, the more likely you are to notice pixellation etc.
Have a good look round on Michael's link to avforums.
Oh.....and shop around on the net - ebuyer etc. before deciding to buy from Curry's, Comet etc...