This is a Royal Mail dirty secret.
When foreign visitors come they may well want to send postcards home; I presume it is a tradition in other countries as it is for us. In tourist locations they could be offered stamps to go with them. Royal Mail allows franchise sales of stamp booklets from news-agents etc. The product that they will have available is booklets of four “Postcard” stamps described as “Worldwide postcard rate.” They are apparently Grey, Blue and Red but I have never seen one—you don’t if you live here. The price of this booklet is £2.68 i.e. 4 x 67p stamps. This is correct if you are visiting from America, India or New Zealand but not if you are from Europe, which must be the majority, they only require 60p stamps. I bet the retailers are either not aware or conveniently forget that.
Mind you, it is not as bad as Spain where once we asked for stamps for postcards and were given some; And for America? Just use two.
There is a similar thing going on with Europe Letter stamps (4 x 88p) which is the second weight, 40g and Worldwide Letter (4 x £1.46) which is the third weight, also 40g but I suspect is less of a problem because the booklets are harder to obtain and most people would go to a post office which, I trust, will advise correctly.