With the advent of electronic price labeling in retail stores we will soon live in a world where the price can change after you've picked up an item of the shelf and before you check out at the front of the store.
I don't think that this would be legal in the UK unless on upwards changes the labels changed a reasonable[0] time before the prices. Unfair Commercial Practices Directive and probably some other laws would probably cover it.
[0] A small shop could probably do it much quicker than a large supermarket which might have to allow for people being in there for an hour.
http://www.marketingmagazine.co.uk/article/1181195/real-time...