How many ml is in a british unit of one shot?Watch
some pubs serve 35ml, but the standard single shot is 25ml
Some places you go in do a larger 35ml or even 50ml shot, but generally it's 25ml. I think there's some law or something about it isn't there? I'll read it next time I'm in the pub, they've got a little A5 sized poster saying something about some act that says you have to give 25ml shots.
So long as you display your measure size clearly, you can serve whatever you want as a shot. 25 ml seems to be what you get in city bars and clubs, but many proper pubs will do 35 ml. At the bar in my room, you are likely to get something closer to 70 ml
You can serve more, but not less.