From a very quick look online, “Grade II” means that the building is of historic or architectural interest. I think it’s reasonable to infer that cleaning graffiti off of a building like that might require materials or consultations that a normal removal wouldn’t, although I have no idea whether £50K is a reasonable amount for that. I guess it would depend on the material, size of the graffiti, etc.

Just a couple of years ago, activists painted a listed building in Norway that also costed about 100k USD to clean

https://www.nrk.no/norge/miljoaktivister-domt-til-a-betale-1...

It's a Grade II-listed building meaning you have to be very careful with it to restore it to its previous state. It's not enough to give it a good scrub, you've also got to ensure you minimise damage to the stonework. There'll also be required paperwork with the local authority to ensure the work they're doing is correct.

It's surprising to me. Spray paint mostly wipes off with cellulose thinners and a cloth.