Not exactly a judge or jury. IIRC a common law assault can go on with only the subjective experience of threat. Hard to prove, but the bar is not objective. See:
https://sentencingcouncil.org.uk/resources/common-offences/a...
That is by no means the only crime that is committed subjectively.
Do you think if I say "hello", and this is my first communication to you, and you feel threatened, and we're in the UK, I will be arrested?
True, although even those crimes (take common law assault) will still be heard in front of a magistrate - there's a process, there are processes for appealing, and it's not just some random police officer with the ability to jail you without process.
I agree that the UK has far too many laws that are more subjective than they ideally should be, but they do at least attach some level of observable and knowable process.