This is a good thing, despite how it seems on the surface.
If it were free you'd probably have 10,000 candidates. If it were £50, you'd probably have 500. Nobody benefits. The city has to spend lots of money and resources handling large volumes of useless candidates and voters do have any idea of who to pay attention to besides one or two huge names and a mass of idiots.
If you can't get 1000 people to give you £10, you don't have any business running anyway.
A large entry fee does a great job weeding out wonks, crazies, and jokers. Real candidates will spend a lot more than 10k through the process anyway.
In the US I voted during the last election to increase the filing fee from $20 to $500 for mayor of Minneapolis. Obviously a much smaller city but the idea is the same.