It seems that I have the answer to the mystery as to why the London based cabion crews seem to be the best.

Had a good chat to one of the Crew in F on a recent flight from HKG. She said to me that the majority of the crew in London have transferred from Aus. It's a highly sort-after transfer particularly amongst the younger crew members. They all of course have many friends living in London. So it's only the crew members with the best reviews / etc that get to move ...

Makes sense when you think about.