Further to my last. The Government intends to carry on with the reimbursement of UK pensioners currently receiving healthcare through the S1 route. This is set out in paragraph 29 of the December phase one agreement between the EU and the UK Government:

Rules for healthcare, including the European Health Insurance Card (EHIC) scheme, will follow Regulation (EC) No 883/2004. Persons whose competent state is the UK and are in the EU27 on the specified date (and vice versa) – whether on a temporary stay or resident – continue to be eligible for healthcare reimbursement, including under the EHIC scheme, as long as that stay, residence or treatment continues.

Regulation (EC) No 883/2004 covers the coordination of social security systems, and allows for the reimbursement of healthcare costs by the UK Government for citizens living in another EEA country or Switzerland in receipt of certain exportable benefits, including a retirement pension. There are some circumstances however where receipt of an exportable benefit will not entitle an individual to healthcare reimbursement, and will depend on the individual’s own circumstances.

It is worth bearing in mind that the withdrawal negotiations are proceeding on the basis that ‘nothing is agreed until everything is agreed’. So the above is all potentially subject to change unless and until the UK and EU negotiators reach agreement on all of the terms of the UK’s exit from the EU. A fuller discussion on the legal status of the phase 1 progress report can be found in the Commons Library briefing paper, Brexit: 'sufficient progress' to move to phase 2.

I hope this is helpful.