Surely you are not suggesting that BA pilots currently towards the bottom of the MSL would be made redundant to allow all of the BMI pilots to come over because their DOJ is earlier?
Google is your friend:
Continuous service means working for the same employer without interruption.
Changes of employer normally breaks continuity, which means that employees must start all over again to qualify for rights
However, there are exceptions.
A trade, business or undertaking or part of an undertaking is transferred to another employer