The greatest British PM of the twentieth century was David Lloyd George. Whenever you or a relative picks up an Old Age Pension, thank him.
However just like WSC he like to interfere with the way a world war was being conducted. However unlike WSC if he had no confidence in a commander he didn't sack him, just undermined him instead - e.g. trying to put the BAF under General Nivelle (not a great success) and withholding troops in 1918, which weakened the army (it was said that there were a lot of troops in the UK). He said in parliament that there were none to spare, yet when the Germans launched their offensive thousands suddenly became available.
I could also add he sold peerages for money to the Liberal Party and probably mishandled Ireland post WW1.
For those reasons IMVHO DLG does not make it into the top ten, but would be in the top twenty for his 'People's Budget', which laid the foundations of the modern Welfare State. However without WSC it might not have lasted post-'40, especially if the UK surrendered to the Third Reich.