Population

I think 'population' is uncountable and therefore it's grammatically correct to say 'the UK's population is'.

I'm sure someone will correct me if I'm wrong!

Hi
Though the word Population is often used as an uncountable noun as Beth says, it can also be considered as a countable group/collective noun (we see 'population' as a whole unit, not as many individuals). When it is considered as a group noun, we use the singular form of the verb, i.e.....IS.
So, as you both said....The UK's population IS....is correct.

The UK's population is?

I think it can depend on what follows, e.g.

The population is wiser than you think. (You are thinking if the population as a whole)

The population are idiots. (You are thinking of the individual members of the population)

There is no hard and fast rule about whether collective nouns take a singular or plural verb.