You can get arrested for making a joke that might offend someone in a private whatsapp group. Doesn't even have to be political.
I think this is the link where the chap asks for legal advice: https://www.reddit.com/r/LegalAdviceUK/comments/1vxkcic/poli...
Since Reddit is now login-walled, can you share what it says?
Reddit often contains completely made up but believeable bullshit, so I wouldn't cite anything you see there. Every single post or comment could be a creative writing exercise to gain karma or sow discord.
However, if you want hard evidence what that post was claiming, I can give you a real citation from 2018:
https://www.bbc.co.uk/news/uk-46106224
> A house linked to an "offensive" video showing a model of Grenfell Tower being burned on a bonfire has been searched by police. Six men [...] were arrested on suspicion of a public order offence and have been released under investigation. A video shared on social media shows a cardboard model of the tower being set alight by a laughing crowd.
https://www.bbc.co.uk/news/uk-46112026
> Can I be offensive in my own home?
> Basically, yes. [...] if you make offensive comments in your home and they are recorded and posted online, it might be possible that you could be prosecuted.