Firstly, I don't think that you would get a contract for just '3/4' months - of course, it is up to you how long you wish to stay with BMI - but bear in mind it will take nearly 5 weeks to train you and sometimes there is a wait to actually go on a training course.
As far as I know, applications are generally for LHR (there are no crew based at LGW) and they are initially to operate on the shorthaul network. Once established at LHR, you would then request to transfer to MAN - as and when vacancies occur. I think MAN is a very popular base and so you may wait for a long time - quite probably more than 3 or 4 months.
Applications for BMI Mainline would not be sent to BMI Regional - you would need to apply to them separately in Aberdeen.
