I also feel this is implausible to the point of ASB. Even during the Heneral Strike in the 20’s there was no stomach for revolution.
So for fun. There is a greater polarisation in UK politics during the 20’s and 30’s. The Communist movement is more popular within the general Labour Movement. Therefore there is no split into two parties as OTL with TTL’s Labour being more militant and hard left. This in turn galvanises the right as the fear of the Red’s is wide spread in a number of circles. This leads to a number of openly fascist MP’s in the 30’s.
By the late 30’s both extremes are using Nazi inspired political violence against each other. The German-Russian pact of August 1939 and the following invasion of Poland caused considerable panic among the British left with some declaring for Britain but more deciding to follow Moscow’s lead and give at best lip service to the British war effort. This angered the more nationalistic British far right and the right leaning Conservative government. It also made any National Government an impossible dream.
When communist insured strikes broke out in protest against the war in November 1939 the Government issued arrest warrants for the entire Communist leadership and sent in the Army against the protesters to tragic effect. Many army units refused to engage and some join the protesters.
Soon armed groups start forming across the country declaring for the Left or the Government. These groups are given rudamental training by WW1 veterans and regular army deserters.
The civil war is bloody but due to the quiet in France the British Government is able to redeploy enough reliable Regular formations from the BEF to break the back of the rebels. They are helped by the majority of the RAF and Royal Navy remaining loyal.
Of couse without the BEF France is quickly overrun by the Germans in May/June 1940. Indeed many French historians still blame Britain for this defeat although the RAF did what it could to help.
When Hitler offered generous terms to Britain in the Summer of 1940, the now very right leaning British Government realising that the nation, scared by its recent civil war was in no position to resist felt it had no option but to capitulate.