Sorry about the delay in replying, chaps - I know how eager everyone is for the answer, it's just that I wasn't at home yesterday...
But yes, it applies to the UK, and thus to Scotland too. The Bridgend site was simply one that actually listed the legal measures.
Whisky, gin, vodka, and rum for consumption on the premises must by law be sold in quantities of 25ml or 35 ml, or multiples thereof. A price list must be displayed showing the cost of the drinks. If any drinks (other than the above four) show a quanity alongside the price, they must by law be sold in those quantities or multiples thereof. But...the permitted description of 'by the glass' does not constitute a quantity, and that means they can legally serve you what they like as a measure.