That's a great question!
I personally think there are some brilliant documents out-there with great insight, advice, and guidance that are really valuable, but often overlooked! I think this is partly because UK Gov isn't very good at promoting these documents and people simply dont know where to look.
Nevertheless, I have decided to produce my top and extensively used 16 UK Gov documents (you lucky people) - literally given the keys away to my kingdom!
My top 16 UK Government project & programme guidance documents
1. Cost estimating guidance
2. Estimating requirement
3. Benchmarking
4. Survival guide to challenging costs in major projects
5. Should Cost Modelling
6. Cost estimating practices for Ofwat Regulators’ Alliance for Progressing Infrastructure Development (RAPID) projects
Approaches for estimating and benchmarking costs for large scale water infrastructure projects: Download from gov.uk
7. Project Routemap - Setting up projects for success
Download project routemap guidance from gov.uk
8. The DECA: Understanding challenges in delivering project objectives
9. Construction playbook, those from or working with contractors
10. MMC Guidance
11. Transforming Infrastructure Performance: Roadmap to 2030
12. HMT Greenbook
13. HMT Bluebook - Project case guidance
Download Bluebook from gov.uk
14. HMT Orange book - risk
Download Orangebook from gov.uk
15. HMT Aqua Book - guidance on producing quality analysis for government
Download Aquabook from gov.uk