If it is a high value property, then there are french agents/portals that really market heavily to anglo clients. Like BelleDemeures.com.
I don’t know of any UK agents who have exclusive mandates on French properties (Mat might know different!) but a lot work in partnership with French agents. I think since Brexit for a UK agent to work in France they would have to have a French branch.
So I would just go for a French agent that targets international clients.