You could try Herts&Essex.
I shoot 357mag on the indoor range.
I don't about Fac on the outdoor airgun section though.
I have not been there but we are entitled to go to Andrewsfield outdoor 50m range with rimfire,pistol calibre rifles etc.
Bryan.
I believe you may only connect the cooker yourself if bayonet fitting,so Gassafe engineer is needed to fit the female part.
Also be sure that the cooker is jetted to run on the gas you are to run on.
Bryan.