It's why all bars will have a little plaque saying what size their shots are. Almost always 25ml these days, but 35ml was common in many places. You're allowed to serve shots of either size but not in the same pub.
( edit: Better link: https://www.gov.uk/weights-measures-and-packaging-the-law/sp... )
However in Scotland two sizes were common, a fifth of a gill (slightly more generous than England) and a "nip" or quarter of a gill (a lot more generous). If you're used to ordering a "nip" of something and now you get a lot less you'd be very angry! So the 35ml option is there for the kind of Scottish or Irish establishment which would have been used to these larger measures rather than either try to keep the gill (which is a stupid unit nobody else needs) or anger drunk people.
I wouldn't be surprised if the "Make a sign" difference was to allow licensed premises to gradually shift to the more profitable, smaller, size. Maybe you change the place you own in Glasgow to 25ml first, and if the locals don't kick off you can try Aberdeen next, otherwise try again in a few years.