It's a bit of a grey area, if the mistake price is someting it has previously been sold at, i.e. sale item has wrong price on it that hasn't been removed, then the guidance is the retailer should honor the reduced price, or withdraw the item from sale nationwide for the remainder of the day, but if it's an obvious mistake, such as a hotel room for £1 and that room has never been sold for £1 before, then they don't have honor it, a company can stick whatever terms they want into a contract, but if it doesn't comply with the law it doesn't mean squat.
Boots have a 2for1 promotion on Pampers newborn size 1 or 2 nappies at the moment, it's plastered all over the store, but it doesn't come up on the tills and the staff at my local store wouldn't honour it until I asked for the Manager who promptly told them they had to. (Don't ask, I was buying them for my sister but even though it wasn't my money, I wasn't letting them get away with it).
If you can renew at the lower price, do, it's better to ask forgiveness than permission.