I've travelled a lot around the UK and there are some absolutely gorgeous places to see, from villages to cities and everything in between. There are tourist attractions throughout the UK, however there is also a considerable portion that does not have any. Areas within the UK that have attractions generally get better investment - London being one of them.
London is also the largest city (home to 12.5% of the UKs population) therefore it needs the infrastructure unlike a small village of 300 people. It's not as if the entirety of London is wealthy and you would be deluded if you think that is the case. There is huge disparity between the privileged and the poor, with vast areas of immense deprivation in the city. These areas do not always see the investment you speak of, which is unfortunately the same throughout the country.
London sees the most tourists - approx. 25 million a year, followed by Edinburgh - approx. 13 million a year, then Manchester and Birmingham. There's over 10 million more people visiting the capital.
Whether you like it or not, tourism is a factor in government investment. At least when it comes to infrastructure.