How junior is junior?
Everything is relative. You will join BA with a seniority of around about 4000. It is not likely to rise particularly quickly. While the lucky ones who beat the 55 retirement age will begin to hit 65 next year, there are only around 300 or so of them and there is another big stagnation bulge behind them. By way of example, I am a LHR SH Captain, have been a Captain for 8 years and if I were to get a LH command next year, I will be in that seat for at least 20 years (or until I get fed up).
Command seniority generally appears as follows:
LGW SH 2200
LHR SH 1900 (no significant vacancies currently)
LHR LH 1100
There is some variation in these numbers with some parties suggesting longhaul commands will move to about the 1300 level. As time goes on, that is likely to be the case as the acquired BMI LHR slots move from shorthaul to longhaul. Obviously that will be at the expense of shorthaul commands. In theory, the remaining LHR SH commands are likely to get more senior however there are some productivity changes coming which may make them less attractive to the senior LH SFOs who may just hang on for a seat swap. As things stand, there are approximately 300 pilots in that position currently.
There had previously been secondments to GSS (747-8 freighters, now defunct) and to Open Skies. To my knowledge there are literally one or two secondees to OS. Again, their current seniority is at around the 2000 level.