NHS England is likely to recommend their version of this EU Patient Summary - European Interoperability Specifications for Digital Solutions in Healthcare which isn’t an API (it really needs an API for implementation), this uses the FHIR version of Clinical Document Architecture (CDA) and is aimed at cross border data sharing.
It could be used for cross-sector data sharing ….. be warned it is a quite involved way of using FHIR, some of the basic API’s in the previous EU spec have more simple methods of sharing documents e.g. PDF in the document exchange API.