That could be because they are more popular (given the price), or because less people buy insurance for them (the scurge of the extended warranty tends to affect those people who just go into Dixons and buy the dearest set they can find without realising the cost or worthiness).
One also has to consider the fact that is it worth getting a £259 set repaired for a cost of £150, probably not, but how about a £699 set, well you aint going to buy a new one at that price.
Both Sony's Bravia and Panasonic's Viera set's have had a History
A lot of manufacturers include 3 year warranties nowadays, I know as standard, Samsung panels themselves have a 5yr OEM warranty on them, so thier is little to lose in a manufacturer offering a good warranty.
At the end of the day, if you buy a big brand cheap, you will get an out of date and less reliable model then their flagship products (and may even end up with a lesser warranty as a result). If you buy a cheaper set with a good warranty, at least if the worst case scenario does happen, you will be sure you can send it back for repair.
To be honest, a lot of the sets that come on to eBay are actually on there due to owner error or stupidity, not properly mounting VESA mounts etc, or surging the sets etc. My "Dual" LCD actually has more features then the "LG" set, despite LG Philips being one of the better brands and costing twice as much.