I dont know about the Trafalgar arms fair but i went to a vintage and classic arms fair at Bisley a few years ago, i spent hours drooling at all the lovely muzzleloaders, rifles and yes there were air guns too.
Do not go unless you have a large quantity of the folding stuff, if like me you are potless dont go because you will want to weep at all the lost bargains!
I vowed to return another day with a large wad of dosh, as yet i havent made it!