There are several vegan Thai/Chinese-style buffet restaurants in London which serve some hot drinks with unusual flavours such as "lemon plum". OK, that's self-explanatory, but one of the others is called "hawthorn plum": (a) what is it? and (b) I quite like it - but is it available in the shops?

I thought hawthorns were just a plant, but checking Wiki apparently there is a Chinese Hawthorn, so the drink is probably a mixture of these fruits and plums. Maybe try a Chinese wholesalers?

Well spotted - I'd already tried Google without success. Maybe the fruit of the Chinese hawthorn is called a hawthorn plum?

Maybe, although it is strange that you say they also sell a "lemon plum" variation. I think your best bet would be to ask next time you are there!