Fighting back against supermarkets

Asda were last week selling packs of pringles at £1.00 a tube with big yellow "Roll back" shelf labelling.
This week Asda are selling the same tubes of pringles at £1.50, once again with a big yellow "Roll back" shelf label.

How can a supermarket be allowed to sell a product at 50% higher in price than it was just a week previously and it is still able to advertise it as being "On sale" and not have action taken against it by trading standards?
Anyone would think trading standards wasn't doing its job properly?