Err aren't the two major airports for the UK in London, with direct connections to alsmost every major city in the world. That would make it pretty hrad not to vist London if you were travelling to the UK
Glasgow, Birmingham and Manchester are big enough that if people wanted to visit Scotland, the Midlands or the North and not bother with London then they could.
The thing is that whilst the UK has some really nice places, it has few that would make people travel thousands of miles to visit. Especially considering that foreigners do not like our food, even we do not like the climate, we're relatively expensive and geographically isolated.
The big draw card is London. Edinburgh, Oxford, York etc also see a fair of trade as does Manchester for entirely different reasons but I don't think very many people would come to the UK just for Oxford.