I love visiting Canterbury, which has some beautiful walls which date back several hundred years, and I’m sure if a foreign visitor came to the UK and defaced one of them with graffiti I’d be suitably pissed off, but in terms of appropriate punishment I would say some form of reparation (a fine to cover the clean up) plus some hours of community service would be suitably condign. Sending them to prison will serve no useful purpose.
It's worth remembering that less than two hundred years ago theft and a number of other offences related to assaults on property were punishable by death. The Black Act of 1723 created 50 capital offences for acts of theft, which was designed simply to protect the property interests of the ruling classes. More recently, the birch was administered to those who dishonestly appropriated property belonging to another. This offence falls far short of theft as there is no attributable loss to any individual, especially a vulnerable one.
I'm sure such forms of punishment will give the usual suspects on here a raging hard-on, but to me, a society that severely punishes assaults on property is one that has its priorities completely wrong.