I'd have probably gone and argued with another member of staff about it on another day.

You are a bad boy. But the real question is whether separatign one item from the set devalued the set. Sometimes, for something antique or collectible, that could make a big difference to its price and desirability. Also, whether the set was otherwise complete in the first place.

However, as they only wanted £5 for the whole thing, it probably wasn't that critical. Couldn't you just bring yourself to buy the set. And get lots of mugs? 😉

ps. Glad you donated £2.