Not free, at the cost of the beauty of the vandalized area

They can't. It's a listed building. They're legally obligated to remove it.

Half this cost is related to security to actively stop people looking at the art work. If it's not acceptable for banksy to cost tax payers £50k to remove the graffiti, why is acceptable for the powers that be to waste another £50k on security to stop anyone seeing it?

And if that ruins the character of the building, why is the fence acceptable?