Reddit reviews Britain Since 1918: A Political History

I can recommend a few for modern politics.

For the Tories you are probably best off reading Tim Bale's Conservative Party from Thatcher To Cameron. For New Labour, with the caveat that it's perhaps a bit too Shakespearean and focuses on personal struggles, Andrew Rawnsley's The End of the Party is a good read. David Marquand's Britain Since 1918 tries to do politics over a broader scope, though I think his analysis is a little superficial. There are other recommendations in the subs' media library.