As an S1 holder, you have the right to join the French reimbursements system under the same conditions as other French residents. This includes full (“free”) reimbursement for treatment for certain affectations de longue durée (serious long-term conditions), such as cancer.
The English-speaking helpline for the French health system said you should not use your Ehic for this but should visit your local Cpam office as a matter of urgency with your S1 form (for British state pensioners or those on certain long-term disability benefits).
They say to take a doctor’s letter from the UK indicating the urgency of your condition to speed up the dossier.
This is because the Ehic is normally for urgent new health problems that crop up while on a visit to a country, rather than healthcare for a pre-existing condition.
Brexit complicates this because the S1 is an EU scheme. If there is a deal, then nothing should change. If there is a no-deal Brexit, France has said there would be no change for two years pending discussions with the UK on a bilateral healthcare deal.
The French authorities have not confirmed what happens if no agreement is reached but it is likely that those with income over a set level would have to pay an annual fee to join France’s Puma healthcare system.
This fee is currently 8% of “capital” income over €10,131 (currently state pensions are not included in this calculation).