A good start would be to use geoip. It's not perfect, but it will almost certainly be enough to make UK happy (the same happens when detecting European for GDPR purposes).

Lmao, why would a web server operator need to care where their clients send requests from? Imagine if half the countries in the world required this, each with distinct requirements on how to handle traffic from their jurisdictions. Insane. Relieve us of the misery of acting as though OFCOM’s requests are reasonable- they are not.