I had something similar happen (not in UK though). A guy followed me home from lidl. Really bizzare. He saw me look at ice-cream and kinda circled around me a few times. Anyway I got home, was putting my shopping away when I heard a knock and he was there, standing holding bloody ice-cream I was eyeing up earlier. Literally said something along the lines of 'I saw you looking so I bought it for you'. It creeped me out so much, I panicked and tried to close my doors but he put his foot and his hand in between. In my panicked state I didn't ask him to move it or wait around I literally started slamming the door into his body parts until he removed himself. I shut the door and called the police.

Police never found him but also considered it reasonable force I guess because I didn't get in trouble (but again not in UK). I would not be surprised if he had a broken finger or two.