Not Tony Robinson I know, but the obvious thing in South Wales would be the beaches after busy sunny days. Used to do that as a kid to great profit, although of course there were no metal detectors in those days.
Research ancient routes, buildings and events but at the risk of upsetting people they’re pretty few and far between in that area.

I’ve often thought it’d be a great idea to use another modern thing - a drone- to explore remote untrodden areas for unnatural features and again , thinking of the particular area, what about mountain tops between valleys. There’d have been farms and probably monasteries etc long forgotten and not recorded. Also there’d be ancient tombs up there.

Speak to the RAF and USAF about aircraft which went down or missing in such locations.

It’s a most interesting proposition and I shall give it some thought.