MJR - basically you are not going to go straight onto a 747 or any 2 crew longhaul ETOPS aircraft until you have unfrozen the ATPL in the UK at least with 1,500 hours. Certainly people who have spent 3-4 years within BA may have unfrozen their licence, but the fact is that to convert to the 747 is highly undesirable at that stage due to the poor position that such a person would have in the bidding for routes, due to low seniority.
People have often gone to other airlines that are usually Far Eastern as 'cruise pilots' that means that they are not main operating pilots but operate under supervision of another as a 'heavy' pilot, ie not performing takeoff or landing duties but only cruise midflight duties often on very long haul. These have been in the past those from highly though-of flying schools with low hours who were on BA sponsorship in the early 90s, but failed to gain employment in BA due to a block on recruitment.