What I mean is, if you join as DEP on the 777, and then in a years time, someone who joined 4 years ago and has been on e.g. 737 moves onto the 777 will they be able to out bid you ???
Woodpecker is correct. Bidline is run based upon 'global' (as you put it) seniority. Anyone senior to you on your fleet (based upon date of joining BA) gets a greater choice. For example, I spent four years in the regions before moving to LHR. When I joined the 777 fleet, my seniority meant I joined about a quarter of the way up the list.
If so, you could be at the bottom of the list for a LONG time, and even worse I guess on the 744.....
Yes indeed, although that depends on how many people join after you. However, there are very unlikely to be any vacancies on the 744 anyway, so that wouldn't be too much of an issue for a new joiner for the first five years.