It depends It is complicated.
. If you are are eligible for French health care and a carte vitale (a healthcare card) and if your diabetes is treated by some form of medication then once you have chosen a GP (medecin traitant) your doctor can fill in the forms for it to be treated as an ALD (affection longue durée)
This means that treatment, lab tests, doctors visits connected with your diabetes will be almost totally reimbursed at 100% You may sometimes have to pay upfront. but you will get most of the money back except for a small ''franchise' (50c or 1€ for each box of pills, visit to doctor or lab etc. )
The 100% reimbursement does not cover anything unrelated to diabetes
. if you needed antibiotics or broke a leg or needed a filling then you would not get all the costs back. Most people have a top-up insurance to deal with these sort of costs
You are eligible for French health care through working , or running a business and paying cotisations .
If you are of
state pension age and receive an OAP from the UK then you need a form S1 from the Dept of work and pensions. This will enable you to affiliate to the French system with the UK government paying your costs to France
.
If you are not of state pension age and are not working or have not set up a business, then it may not be possible to join the French system until you have lived there for 5 years legally. If this is the case then you would need to have private health insurance for five years at which point you become eligible to join the CMU as a permanent resident. When allowed to join, you would have to pay cotisation of about 8% of your residual income.(basically after the households personal allowance)
This would continue until you are in receipt of a UK state pension.(when you can get a form S1
(I say may, because the situation is in flux . A few years ago they stopped early retirees being able to immediately join the CMU through paying cotisations People had to get private insurance until the 5 years was up. There has been some pressure from the EU on France about this and the situation seems totally unclear http://www.frenchentree.com/living-in-france/healthcare/healthcare-rights-for-early-retirees/
I don't know of anyone who has come here as an early retiree recently without having to get private insurance.
If you are in this situation, don't move without healthcare. I know someone who has had to return to live with friends in the UK whilst undergoing cancer treatment. She has no healthcare rights here and they can't sell the house.