Not a lawyer not legal advice.

Not correct. The price on the 'Box' is the invitation to treat [0]. The price you pay in the UK is the verbal Offer at the checkout. They may/should sell it a the RRP, but it is not correct in law.

[0] https://en.wikipedia.org/wiki/Invitation_to_treat

Or less, as the store is allowed to discount (officially the reverse: the manufacturer is not allowed to set prices of anyone except themselves, so if a store takes almost no margin or even sells below price, the manufacturers is forbidden to even talk about it.