The big multiple stores are VERY careful to follow the letter of the law, I know because I used to work for them. As they have a higher profile they tend to get taken to task more often by customers so it’d be difficult to get away with it.
However “price establishment” is common. Go into any DFS or similar and they’ll have some very expensive sofas around, maybe at the back of the store. No one will buy them (and if they do huge profits!) but they are put on display for 28days then reduced.
Not all the stores have to do it as it has to be a certain percentage of stores that have to stock it at the higher price. We used to get strict instructions to have x on sale from a certain date OR ELSE!
Bit more complex nowadays with the internet but it should be easy to track PX as they are essential one store.
They should keep records of what each item has been priced at for historically so they can prove they are complying.
Smaller stores or one man bands tend to get away with dodgy pricing as they just aren’t in the spotlight as much.
If anyone genuinely thinks that PX aren’t following the rules then report them to trading standards, we used to get them in our store occasionally to audit our pricing and they are very thorough.