I don't know of a single case (and don't believe anyone can point to one) where people have been arrested for simply criticising politicians. Has the number of incidents risen over recent years? Yes, and while this might be partly explained by stricter legal approaches, I suspect it's much more to do with a drastic rise in far-right activity and a consequent feeling among many that they can now say/do whatever they like with impunity (including making threatening and inflammatory remarks about minorities, and so on).
https://sentencingcouncil.org.uk/resources/common-offences/a...
That is by no means the only crime that is committed subjectively.
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.