Stuart 
I asked HMRC what open source software there was for "Making Tax Digital". Their answer: use their software-finding tool. I'd asked them the question because their tool doesn't make that information available.
Does such a thing exist?
TheZorseSimilar problem in Canada, as someone else mentioned.
The government provides a list of software that is approved based on whether it has the right functionality, but makes sure to note that it is up to the user to check the privacy policies. No mention of the fact that having a policy and honouring it are two different things.
Only one program works with Linux. None is open source. Many, I worry, only exist to gather data.
It's a mess, and for the relatively simplistic nature of tax software, the government could easily provide its own, fillable forms on the same website used for all other tax stuff.