Have a look at this poster found in PC World.
Got to the checkout, it said £199.99 and the Manager wouldn't honour the advertised price. He has a legal right to refuse the sale but it suggests that PC World will cherry pick what they will accept and what they won't. This is clearly not a mistake by the local store, but by head office marketing who produce these fliers.
I wonder how many times the posted price has actually been higher than it should have been - and how often did they volunteer to correct it. I was buying several other items as well. As a result, I bought nothing.
I'm going to contact their head office and see how far I can get.