Perhaps an attempted Communist revolution, triggering civil war, during which the Germans are able to launch an invasion. Shades of Saki's When William Came.
I think any one-party state would suck, regardless of position on the spectrum of ideologies.One party dictatorship in Great Britain by either the Labour Party,Socialist Party or Communist Party.
"What if Gordon Banks had played" s pretty dystopian, as is "For All Time"
I think you would have to go back to the early 20th Century to make it work.
It would require a single party government, a couple of "Emergencies" and WW1 and I think you could have a defacto dictatorship by 1923.
From there, even if it's a centreist Imperial dictatorship, I think you'll see enough agitation from both left and right, as well as significant nationalist agitation in Ireland* & India. I think you'd also end up with less direct agitation from Canada, New Zealand and Australia, Kenya & Southern Africa.
SNIP
"What if Gordon Banks had played" s pretty dystopian, as is "For All Time"
Then again, despite his protestations against nationalism, there is an observable strain of nationalism, bordering on xenophobia, in Orwell(especially noticable in his essays), and one can imagine him just lumping "bloody yanks and bloody metric" together as part of one foreign onslaught destined to overrun England.