I think this is not correct, if you have less than 20 seats and most Cargo A/C dont have more
, you dont need a cabin crew, all the emer proc. will be done by the cockpit crew. See DO 228, DHC-6 etc, no cabine crew.
Gemini accepts jumpseats, but I dont know if they fly to the UK, check their website
http://www.gac-cargo.com/