From personal experience as a UK citizen living in France, I’ve signed legal forms in the company of a local notaire who has signed and stamped as a witness and the forms we’re then accepted in the UK.
Having dealt with UK solicitors in they pay, my only surprise is that he actually laughed when I asked what the fee was: “Frais? bien sûr que non - c’est un service d’honneur”. When I came round about half an hour later, the answer was still the same…
However, that was before Brexit so, with services not being included in the agreement, the situation may have changed.
If I were in your situation, I’d probably give AgeUK a call to see what advice they offer.