Right, but not all drivers operating a vehicle in the UK are required to pass a UK driving theory test. Virtually all foreign drivers can arrive, show up to a rental car place on their foreign documents and just drive out. While in the best case scenario a driver may have a UK licence and passed a corresponding theory test, this is not guaranteed. There's zero requirement to have sat that test for almost all foreign cars entering the UK via train and ferry too.

As a counter example, a UK person visiting the US would have almost zero difficulty understanding any of the speed signage, as they all incorporate the actual speed limits. Similarly, there is zero expectation you have passed the theory element of a US drivers licence. The US has roads with different speed limits for different vehicles too and still manages to get clear signage with numbers.

Good signage can be clear regardless of any tests, virtually all other speed signage in the UK incorporates a number. Speaking from experience, even those who have sat a theory test can often not really understand that sign or forget its meaning. There's not even any guarantee a licenced UK citizen has ever sat a theory test, given those who gained a UK licence before the introduction of the theory test in 1996 have generally never been required to sit one. The pre 1996 "theory" element was some random questions from your tester during the practical.