i've bought lots online, no problems at all.
pricerunner, kelkoo etc are a good starting point but they don't always find the best price. i've found it best to research what you want by going round the shops, reading up (as per Mr M) then when you narrow it down to a few model numbers, just search on these using your normal search engine.
it might take a few hours, but on a plasma or LCD tv, you could be saving £100's...(when we bought our 32" widescreen - we got the tv & stand, dvd home cinema & video player online for the price curry's wanted JUST for the tv!)
try - hughesdirect.co.uk
unbeatable.co.uk
digital-point.co.uk
rgbdirect.co.uk
happy shopping!