This is actually the old law and now you need to prove that the RRP is "realistic" which usually means it has been sold at that price for more than half the time it has been on sale.
I suppose you can just pick the slowest 15% of days, then. If a price was randomly jacked up on a Tuesday, but nobody was around to buy it, was it really raised?
Source: https://marketinglaw.osborneclarke.com/advertising-regulatio...