And more #PublicDomain volumes from Google:

Parliament sessional papers 1909:
https://books.google.co.uk/books?id=wPdGAQAAMAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

The British Parliament:
https://books.google.co.uk/books?id=PEwvAAAAYAAJ&printsec=frontcover&redir_esc=y#v=onepage&q&f=false

Bodø-Saken: British Diplomatic Correspondence Relating to the Bodø Affair
https://books.google.co.uk/books?id=lLdJAAAAMAAJ&printsec=frontcover#v=onepage&q&f=false

And more #PublicDomain volumes from Google:

British Documents on the Origins of the War, 1898-1914:
Vol 1
https://books.google.co.uk/books?id=nEAOAQAAIAAJ
Vol 3
https://books.google.co.uk/books?id=9tpmAAAAMAAJ

Notes on the Land & Air Forces of the British Oversea Dominions
https://books.google.co.uk/books?id=LVo2j73mzK0C

1920s #PublicDomain UK state volumes on #Africa:

Reports on Tanganyika
1923:
https://books.google.co.uk/books?id=iw9SAQAAMAAJ
1927:
https://books.google.co.uk/books?id=kHvMJDks7yoC

Handbooks on African territories, 1920:
https://books.google.co.uk/books?id=BoszAQAAMAAJ

More #PublicDomain vols released by Google:

1921 Welsh Census reports:

Glamorgan
https://books.google.co.uk/books?id=yTTRHznn0ZoC&printsec=frontcover#v=onepage&q&f=false

Merioneth and Montgomery
https://books.google.co.uk/books?id=o1jDNcnm5rMC&printsec=frontcover#v=onepage&q&f=false

Brecknock and Radnor
https://books.google.co.uk/books?id=b4ti4LL8WRIC&printsec=frontcover&redir_esc=y#v=onepage&q&f=false

More #PublicDomain 1921 Census reports released by Google:

General Tables:
https://books.google.co.uk/books?id=2weiOeZQpK0C&printsec=frontcover#v=onepage&q&f=false

London
https://books.google.co.uk/books?id=c-xQk9bnoFUC&printsec=frontcover#v=onepage&q&f=false

Kent
https://books.google.co.uk/books?id=FgnBh0Zs-I4C&printsec=frontcover#v=onepage&q&f=false