The 23 strong Britannia fleet was shared by 99 and 511 and used for training by ATSB.
When I joined the fleet in 1966 the conversion course consisted of 12 weeks of ground school at Filton followed by another 12 weeks of ground school, simulator and 14 hours flying with ATSB at Lyneham. Then we were rewarded with a two week round the world global trainer carrying 2 ATSB crews and two trainee crews. We routed via Akrotiri - Bahrain - Gan - Tengah - Kai Tak - Guam - Honolulu - Hickham - Offutt - Gander - Lyneham.
I don't know when the end of course Global Trainers started, but I do know they were still going strong when I left in 1974. ATSB crews occasionally operated down the route without trainees to keep their hands in.