The normal procedure is giving £100s to a geezer who's done a one day pat testing college course
You could try sweet talking local shops that sell reconditioned electrical stuff - or you might find a charity shop that sells old electrical stuff who could do it in return for a modest donation.
Not a recommendation... but anyone just can buy PAT test passed stickers and if the thing they're stuck on is not obviously charred, knackered or frayed there'd probably be no questions asked.