OldFrill
As per my previous responses l believe Banksy has enhanced this sign by £500,000 which l assume now belongs to whatever authority owns the sign. I think any attempt at prosecution due to the sign having been 'defaced' (as this is the term used in the OP) ludicrous. As I've also previously stated no other street artist can achieve a similar enhancement.
I've already explained that your opinion and your beliefs are not relevant to matters of law. Just as mine are not relevant. No matter how many times you tell us what you believe it won't change the law.
Then you have to consider the unthinkable.....what if this was a copy cat artist and not Banksy? In your opinion would it still be an enhancement if he was a nobody and had not actually increased its value? Or does it actually matter as long as people think it's done by Banksy?