I am a pensioner. If I apply for the S1 form, will I still enjoy full access to the NHS in the UK?

Yes. people living in the EU whose healthcare costs are funded by the UK under the current EU arrangements (such as those with a UK-issued S1), will be entitled to free NHS hospital treatment in England (primary and emergency care), should they return to the UK. It is only secondary care (hospital care with specialists) that could be chargeable. This is the same as for S1 holders who live in the UK.