"i'm a biker living in london UK'
I trust that forum's admin gave that user's IP address to Scotland Yard - certainly to it's Cybercrime Section - and let matters take it's course.
Crapital F(uckup) here would then find himself in the ultimate 'Trolling' exercise: when he's haul-arsed off by London's Metropolitan Police; even if it is just a 'joke', he's gonna soon find himself sobbing, soaked in piss when he's another of the five trolls a day arrested here in the UK.
Can you say 'Malicious Communications Laws'? Because the Met Police, as per the FBI & Jake Brahm, don't have a sense of humour. When it comes to peoples' lives, they're not paid to have one.
A forum post by someone who says he's a member of ISIS, and says he'll plant bombs throughout London. You, Crapital F(uckup). No Difference.
Remember: Malicious Communications laws.
No Fun Allowed.