Guesswork but I suspect they've split it as follows:

Person 1: Ramen - £10 (being max allowed)
Person 2: Salmon+Still - £9.25 (being 50% of £18.50)
Person 3: Meatballs - £3.75 (being 50% of 7.50)

Total of £23. I'm assuming that "Large Still" is still, mineral water or something else that is allowed. The rest of bill being alcohol or cover charge and so not in scope.

Edit: I'm now wondering if an establishment sells a mixer separately does that become in scope?