rest of europe already have track and trace software that could be used here. This government decided they wanted their own as they are more concerned about Brexit than the heath of the nation. We've already seen this in action with reference to COV19. The Europe app is time limited the UK app is not.
Five seconds googling, and you will find that the framework used by most other countries, including the many outside of Europe, is an open framework developed by Google and Apple. Who are, erm, American. There is no 'European' app per se. This has nothing to do with Brexit / EU policy.
I think that NHS IT are batshit crazy not using it, but its nothing to do with Brexit for Gods sake.