They have said the server code will be coming soon. The intention has been to release the source code for the app since it was announced. I doubt this is a last minute political decision since the UK government already release a huge amount of their code as open source.
The project isn't perfect - but consider that it's been put together in a month, it's still beta and their focus has been on making it work, not making it easy to build.
Frankly, the level of transparency from the NCSC and NHSX has been far ahead of most governments and private organisations. You can disagree with the contact tracing approach, but if all government work was done with this level of transparency we'd be in a much better position than we are now.
The project isn't perfect - but consider that it's been put together in a month, it's still beta and their focus has been on making it work, not making it easy to build.
Frankly, the level of transparency from the NCSC and NHSX has been far ahead of most governments and private organisations. You can disagree with the contact tracing approach, but if all government work was done with this level of transparency we'd be in a much better position than we are now.