I think you'd be alright wearing similar gear as long as you don't have any patches that actually say 'POLICE'.
Would be funny to have the same loadout with patches that say 'PLOD' or 'PIG'
Try to be more POLITE.
Impersonation is a curious offence. The offence of actually
wearing police uniform requires intent to deceive, which won't apply if you're only doing it for dress up at an airsoft site.
However, mere possession requires you to "[prove that you] obtained possession of that article lawfully and has possession of it for a lawful purpose". No specific purposes are mentioned. Film or theatre would be the obvious one. Airsoft, dunno, best of luck.
https://www.legislation.gov.uk/ukpga/1996/16/section/90
An aside, there was a Scotch stripper who went by the moniker
Sergeant Eros who used to wear realistic uniform and gear, with "STRIPPER" on the back. He was arrested and prosecuted
repeatedly for doing so, and for possession of an offensive weapon (his baton and fake PAVA spray, which is where I got interested in the case) despite Sheriffs sending the police and Fiscal away with bees in their ears several times.
At first I was all "Boo, massive over-reaction, poor show, petty harassment, power to the people". However, it turned out that he was a proper fantasist and they finally did him for
fitting blues to his car and pulling folk over.
But that does show that it's more what you do with it than merely possessing it.