Isn't the solution here basic? Why not simply put bandages on the sores before getting into bed?

I don't know UK law but it seems to me arguing this is a disability in the US under the similarity intended ADA would be a stretch.

Having said that, the past can't be changed and the OP is already in the situation. I don't think the hotel acted appropriately IF the OP knows them as well as they say. I would think basic customer service would have some sort of discussion or warning before simply sending a bill. And explanation would probably have gone a long way to minimizing the anger from the customer. And maybe try a more reasonable cost rather than trying to profit.

While I don't agree with the OP it certainly behooves the hotel to be far more empathetic in how they handle a situation that will obviously be sensitive to a guest.