The vast bulk of those cases are about online harassment, usually against former spouses, public servants, etc. If you are aware of a case where an individual was arrested for just expressing their opinion you are welcome to provide the evidence. Until then this is just FUD. Censorship is bad, protecting the rest of the citizens from harassment is the kind of thing that is actually useful.

What were they arrested for saying?

Are you expecting me to comb through thousands of cases? Obviously they were arrested for saying legal things, if their arrest doesn't follow a conviction in 90% of cases.

If you're going to claim that people get arrested in the UK for criticising the government, it's reasonable to expect you have an example to hand.

I expect that when you say someone was arrested for speech and it's government overreach (as opposed to a legitimate arrest), you should show us the speech they were arrested for, to back up your claim that it's overreach.