I think, after some googling, I have got it sorted.
The Gas Safe Register states quite clearly that ^We're the official list of gas engineers who are registered to work safely and legally on boilers, cookers, fires and all other gas appliances.*By law all gas engineers must be on the Gas Safe Register.*
Gas Safe Register replaced CORGI as the gas registration body in 2009.
CORGI state that their heritage as the previous GB gas scheme registrar has allowed the organisation to operate from the centre of the heating industry – with good relations with government, manufacturers, distributors and merchants.
Currently the CORGI brand is licensed for use by a carefully selected range of businesses, including Insurance, Certification, Van Sales, Warranties, Underfloor Heating, Telecoms, Controls, Technical Services and industry-related product lines.
I think CORGI works like HomeServe, acting as a central system and operating through local tradesman whom they have approved.