If you want to know what the MoD's latest position on software safety is, you need to get into this site's Standards Of Best Practice:
https://ssei.org.uk/documents/
One of the key aspects with software safety on this scale isn't access to the source code per se, but confidence that the software is safe based on the processes used to develop it and the evidence presented about the product.
Having sufficient confidence (true confidence, not "it'll be ok because, gov...") will be the tricky bit.
sw