I mean… the only thing still measured in stone is babies, and even that's falling out of favour.

Most adults in the UK still measure their weight in stones and pounds, at least in my personal experience, and I have never come across anyone (except those raised in other countries) using either plain pounds or metric units for this purpose.

Adults in the UK who take sport in any way seriously are more likely to quote their weight in kg, in my experience.

Babies weigh less than 1 stone (14 lb), they would traditionally be weighed in pounds and ounces.

From searching "baby weight NHS", it looks like all the advice uses grams, so I assume that's current practise.

Well, I must've completely misunderstood several conversations, then. Which would be characteristic.