That's the problem. Corruption in the UK isn't perceived as such, because they go to greater lengths to hide it. That doesn't change the fact that UK politics are rotten to the core and their politics have a much bigger global effect. Remember when London bankers recently manipulated the stock market and got away with it?
And let's not ignore that Transparency International themselves are somewhat shady. They gave Hillary Clinton an award for integrity and that's only the funniest of their failings.
That's a very simplistic and primitive way to look at it. For one, there are more and wealthier people involved so the costs are shared. Profits are also bigger, because the UK is better connected, so the increased investment costs are acceptable.