Hi Carol.
I think you will be pleasantly surprised at the cost of a mutuelle here in France which is why I suggested Fabien . You can see the thread about him here.
IIRC it is against the law in France to discriminate against people with pre-existing ailments unlike the UK where you most certainly can expect to get ripped off.
We have a mutuelle arranged by Fabien and given that I have a number of ALD issues (affection de longue duree) the cost in my view is extremely reasonable for what we have selected as the level of cover required.
The reason for mentioning retirement is the issue of an S1 from DWP which usually comes with the UK State Pension meaning that heath care in France receives a contribution from HMG which will go some way towards satisfying the requirement to show to the French authorities that you meet the residency rules. We have been fiscally resident in France for 10 years now so this is less important to us as residency rules require a proven period of 5 years. How this may change after the dreaded Brexit is anyone’s guess but you stand a better chance I think if you are in place in France before the appropriate cut-off date 
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