Perhaps you would like to tell us the name of the wonderfully helpful person… so that we can contact him in future if ever we feel in need of his special services…

oooo, you likkle devil you 
If you ever happen to be over this way, and in need of somebody to fix your car, let me know and I can guide you in the right direction. 
Wow … just like the dry cleaners 'all care no responsibility !! glad you saw it before you needed it !
Our RHD Merc first registered in Nov 2003is built to a far superior standard than the modern cars.
Also we are more likely to encounter tractors than HGV’s.
When I used to drive from Stroud to France there was twenty minutes at either end on ordinary roads, all the rest was on dual carriageways or motorways and there is no problem there.
We do a very small mileage and I think that being able to see the edge of the road is a distinct advantage.
We used to drive from Stroud to France too. Lived at Camp until 1994. I don’t know how much it’s changed since then, but Stroud was a very pleasant little town.
Today I received the official letter terminating our cover with Direct Assurance from 15.08.2022. They have already refunded me the latest premium.
The reason for the cancellation of the contract is in conformity with Article L113-4 du Code des Assurances.
When are you going to ask for all your premiums back?
They’re using a condition to cancel it which doesn’t exist as per the other poster’s info about the Napoleonic code. So you could protest and say your cover should continue for as long as you bought it for as surely they are not allowed to introduce new conditions midterm?
And if this is the grounds they cancel on then their action implies they don’t consider it was a valid contract. Which means they should refund all premiums not just the one where they changed their mind. You didn’t just recently move the steering wheel from left to right did you, and there was nothing about this in the conditions.
Perhaps I’m coming at it from the point of view of equity(fairness) and English law but the Napoleonic law would seem to say that too and they can’t have it both ways.
I’d be on to them sharply via their call centre and request an immediate complete refund of all premiums and leave them in no doubt that this is the required consequence of their decision.
A slight diversion (not uncommon on the roads, surely?) but before I got a proper job here back in '99 I drove a transit box van (RHD) to England a couple of times after combing the south coast of France collecting antique furniture for an English dealer. The chap I worked for came from Stroud and the van I drove had the name of a Stroud baker still emblazoned on the side.
We, I had a mate for the heavy lifting, stopped in a small coastal town for a coffee and when we returned to the van there was an old gentleman standing there with tears in his eyes. ‘I didn’t realise you delivered this far’ he said ‘I used to live in Stroud many years ago and that brings back so many memories’. 
I’ll wait until our new insurance policy is in place and then ask for a refund of all premiums paid. Worth a try, but if they don’t agree tant pis, it’s a small amount. 
Stroud is one of the few places in the UK of which I have very fond memories. I hope it hasn’t changed too much over the 27 years since we left.
That article is in relation to an increase of the risk (risque aggravé) and full article is available here => https://www.legifrance.gouv.fr/codes/article_lc/LEGIARTI000006792030/
For a risk to be considered “worse” it should follow objective rules and, unless the right hand drive is mentioned or excluded somewhere in the terms & conditions or on the policy it cannot be considered as an aggravated risk simply because of the jurisprudence I previously mentioned and the fact that the French DREAL backup the “safeness” of your vehicle to drive on French roads by providing you with a French carte grise. I’m certain that you can legally argue their decisions and ask for a compensation as you’ll have to pay highly increased premium following the termination of your policy by an insurance company. As this is becoming a legal issue you should seek legal assistance but I’m almost certain you cannot lose this one as I really can’t see how they can legally confirm the increase risk without specifically naming it in the terms & conditions. For this the first step would be to file an official complain to their services and ask them to get the details (email or postal address) of their “service réclamation”. And then send a letter where you ask them to specifically pin point which article in their “conditions particulières ou conditions générales” allow them to consider that right hand drive is an increased risk. They should respond within 60 days and their response probably won’t be acceptable (from our perspective) so that will allow you to then write to the “médiateur de l’assurance” which is basically the ombudsman and ask him the same thing but then you add that you think they’ve unrightfully cancelled your policy so you seek compensation. All that comes from free except from the legal advice if you use a lawyer to write the letters.
There used to be occasions where free legal consultations were available via Mairies - may be worth a question there… may be an unhelpful time scale though…
Thank you Fabien.
I will follow up on that.