To highlight why the back end source code needs to be released I thought it best to provide an otherwise pretty neutral/complimentary take on the app.
https://reincubate.com/blog/nhs-covi...acing-details/
The key part being:
"Ultimately, the code suggests there's little technical privacy risk from using the app until such time as a user reports symptoms and contacts the NHS to arrange a test. At that point, their test result would be managed by the NHS, though this isn't any different with or without the app. However, at such time as multiple users come into contact, report symptoms and arrange tests using the identifier that the app has provided them, it may or may not be possible to centrally tie them together. That's a back-end question, and as NHSX states on their app privacy report, code is truth in this regard."