Actually no, not true on the extended warranty. Replacement goods must be like-for-like or better from DSG or whatever they're called now.
My old CRT TV (which coped with NTSC as well as PAL) got replaced by a 720p LCD TV which was three times the price. Why you ask? Because NTSC was needed for my Jap Gamecube and the cheapest telly they had at replacement time which did NTSC was the 720p I got.
The same is true for items damaged on delivery - if no identical item is available they must give you an upgrade to something equivalent or better (they will obviously offer you your money back first though). This is why I have a Dell 4K monitor that was three times the price of the Acer 4K screen I actually bought.
PCW / Currys / Dixons additional warranties can be an absolute bargain - but only on 'End of Line' or hardware with unusual specs. Don't take what a shop grunt says, read the fine print and hold them to it (which isn't difficult, rules is rules in call centres ya know :) ).
Yes I did do a stint at PCServiceCall for my sins (many years ago), but once you know the policies on returns and warranty they really are damn good if bought on the right item.
Oh and one final thing, most people don't haggle in the UK, which is just bloddy stupid... If you sign up for additional warranty the sales person is FAR more flexible on a discount from the sticker price -
simply cancel after leaving the shop.