I’ve put a list of open source + standards I’m aware on here Open health sources
I’ve been a bit liberal on what I view as “open”, might need to narrow that down.
As a developer I tend to favour focusing on the clinical side first, it tends to be the weakest part of software development (analysis and design) and explained poorly in the NHS. Ideally this documentation is expansion of clinical pathways from NICE and GIRFT. The WHO SMART Guidelines are a good example of this.
For example: WHO Immunisations provides background for IHE PCC Vol 1 - Chapter 6 Immunization Content Module. This would have been really useful for me as a developer working on Imms over the years - rather than fast ball during COVID when the clinical pathway description finally got to developers!