All
Concentrate on either a) getting in to BA b) Getting through the first 6 months probation / passing TR/line training.
All these questions re bidding are valid but the bidding system is complicated and at this point fruitless in discussion, only to say, you will have no regrets in joining the 400 fleet and BA in general.
You will be surprised how much flexibility you will have in your roster even being at the bottom.
Enjoy