Some things say "built up" , but the legal documents say "Congested" and that's even less well defined, because some readings would say a city centre deserted under current lockdown does not count, and others would say it definitely does. In the end it will need people to be prosecuted and a court to rule on how the line is drawn.
The more detailed background I linked does go into the "built up" vs. "congested" aspect a little, and it does indeed muddy the waters even further. With legal matters though it's always advisable to err on the broadest combination of two definitions, so if one rule says "A" and another says "B", then it's "A *or* B", not one or the other, and certainly not "A *and* B". In your example, that would absolutely rule out a deserted city centre in the UK, which I think most people would agree to be the intent of the wording in that scenario, although there's definitely footage out there - and posted here - that implies that at least some disagree.
As always though, interpretation of the law ultimately falls to the courts, so until there are prosecutions and precedent we're stuck with our good judgement - and we've just seen how that can go, albeit mostly in the court of public opinion.