Well, they probably don’t, to be honest.
However, it seems only the other day that I wrote “No need for a £5000 fine! No need for an inspector!” (can that really have been a whole decade ago?) on the subject of trading standards and pricing legislation, but it seems Tesco have taken it to heart:
Pity it all kinda hinged on us living in some future libertarian Britain. As far as I’m aware, you can still demand the price displayed when you get to the till, and Trading Standards can still levy Tesco a £5000 fine for each incorrect label, unless there is some sort of grace period following the VAT increase. I somehow doubt it given the grasping nature of our overlords, but it is possible.
Update: Apparently they do! Many thanks to the commenter.