I don't think BA want expensive long service CC anymore. The turnover of Mixed Fleet is much lower than BA had expected and planned for.
Much more flexible ie mixed type/ Long Haul/Short Haul mix, and less double night layovers which the old contract crew refused to do.
The change to Mixed Fleet on the Sydney service 777 route alone for example,has saved BA in excess of £3 mil PA,crew cost, hotel, and Allowances.
In my experience a good MF crew are indistinguishable from the old lot, apart from the hats, and they smile more!!