I could never see the U.K becoming anything like this...
Of course it's only a movie but the British government would never become so ''religious'' and they wouldn't be executing anyone. Likely there would be expulsions of some immigrants, especially Muslims and some level of white nationalism but nothing approaching what is portrayed in the film. The British loathed the toxicity of Nazism. Also homosexuals would be left alone in my opinion.
The political structure would just fall back to classic Toryism, NHS would be eliminated and there would be little social safety net. The level of tyranny would not be like what's portrayed in the film (with curfews and strict punishment) but people who really acted up rioting against the government would likely face long jail sentences.
Anyway.. that's my interpretation.