In the 2012 Civil Service Reform Plan (under the coalition government – remember how we used to be critical of them?), there was a general commitment towards making the whole policy-making process more transparent and open to multiple voices (see Civil-Service-Reform-Plan-final). This has been forgotten under subsequent governments (indeed the page for this plan has gone from the gov.uk website, although the 1-year on reports and press release is still there).
The “Principles of scientific advice to government“, published in 2010 are still there, but I notice that it was last modified 25 June 2020.
Added to the relative lack of enthusiasm for Freedom of Information (FOI) rights (e.g. as discussed in this article) and lack of transparency about embarrassing reports (the security report on Russian interference in the last General Election) this adds up to a retreat from transparency.