After the takeover of Cityflyer quite a few years ago the pilots were intergrated into BA (bottom of list, grandfather rights to command etc). However some pilots were found not to be to BA standards and it was decided that in future ALL pilots would go through selection before joining mainline.
I very much doubt if the BMI pilots would be integrated into mainline due to 1. cost savings, 2. unlikely that competition commisions would allow a takover, 3. pilots would be required to go through selection (see above).