BMI cabin crew are not going to join the Legacy crew, that is for sure. There has been very little information about what will happen to the BMI cabin crew, but I feel there is a surprise coming.
Two things may happen; either they will end up on Mixed Fleet, or they will be the nucleus for BA Express. Mixed Fleet is being pulled off of the PHX route now after losing ATL. The future for this fleet due to 900 hour rule, is shorthaul and short longhaul.
I would not be at all surprised if a new longhaul low cost subsidiary is formed at LGW due to the base losing money using Mainline pilots. As it is Willie's aim to have a premium world airline with a low cost base, you can see a BA version of JetStar coming.