Captive bolt stunners don't require a firearm certificate, anyone over 18 can buy and use one.
Brno guns do a spring loaded poultry/rabbit stunner Stereo but its over £100, I'll post a link to them tomorrow when I get on the pc.
I shoot mine with an air pistol now for ease of use, the turkeys I do at night when its dark using a small light, I bring them outside and just do it as they're sitting there. Big cockerels etc I hold by the feet with one hand and lay them on their front, shoot them down behind the rear of the comb
I just aim straight down through the top of the head probably just behind the eyes, never had one not die instantly.
If they flap I wait till the worst of it is over then dislocate the neck by hand, takes all the pressure off doing them whilst still alive!