In the UK, they can't force you to speak, but not answering questions can be interpreted as evasion / "adverse interference".
They do not need a warrant to search your phone. Not providing them with a PIN can be a separate charge, with up to 2 years imprisonment.
They can hold you up to 36 hrs if the police decide so. Up to 14 days without charge under modern broad "anti terrorism" laws.
Also note the absence of large scale destructive anti-police riots in the UK over the past decade.
The UK have about 13,000 arrests per year over social media etc. Without litigating all 13,000 of them for validity, that's about 185 microliberties.
Birth control is about 1-5 micromorts.
So you're 30x more likely to be arrested for a social media post than death by getting out of bed, and 55x more than being killed by police. And it's in the same range as taking birth control.
Realistically, these are population estimates. Your actual risk of dying by police is highly correlated with the number of violent interactions you have with them. If you're a getaway driver or a bank robber vs SW developer, you probably have significantly different risk profiles.
On the other hand, edgy social media posts are potentially much more diffuse across populations.
That's the figure (slightly inflated) for arrests under both the Malicious Communications Act 1988 and Communications Act 2003. That's not reasonably described as arrests for 'social media etc.' It's quite likely the the majority of those arrests have nothing to do with content that people post on social media.
As for police killings, you are comparing an event that’s extremely unlikely to happen with an event that’s multiple orders of magnitude more likely. Speech offense prosecutions in UK are more common today than political prosecutions were in post Kruschev Soviet Union. The only reason UK has fewer political prisoners than Soviet Union had is that the sentences are shorter and typically non-custodial.