The thing is for most shops with the values talked about it isn't worth the bad press for the sake of a few quid here and there. It's only where online retailers sell things like X-box's for £2.99 instead of £299 and it goes viral that the law is enforced.
No trouble - Sadly one of the things...
https://en.m.wikipedia.org/wiki/Pharmaceutical_Society_of_GB_v_Boots_Cash_Chemists_(Southern)_Ltd
Here's the case law. Slightly different subject but he principals are the same.