That kind of stuff would be in the digital service manual.

https://beta.nhs.uk/service-manual/

Design Principle Six says:

> Test your assumptions

> Design and test your work with real people. Observe behaviour and gather evidence. Work with subject experts and existing research. Do not rely on hunches.

The section on accessibility has this: https://beta.nhs.uk/service-manual/accessibility/user-resear...