Setting up as a Surveyor

I don't know whether you need official qualifications/membership of an association but you definitely need insurance cover - you mentioned indemnity

Sometimes you can't get cover unless you're a member of a professional association. Other times you can get the cover but it can be cheaper if you're a member of a recognised association.

Mrs S couldn't get cover for some of her therapies without professional affiliation.

It should be easy enough to find out.

You seem to be restricting yourself too much to make it only historic building, unless you intend to have it just as a sideline. You'd need to advertise in specialist journals to be able to enhance your chances of getting to your market. I suppose there must be a simpler way nowadays by publicity through the internet.