If you have increased the number of commands by 100 out of the original 1500, then the command pool has increased by 6.6%. If the retirement rate is ~5%/yr then commands generated goes from 75 to 80 commands/yr. Unless of course the the bmi captains are comparatively younger than ba captains. Also the excess bmi commands mean any retirements of bmi captains whilst there is an excess will be given on a ratio of 1:1 ba:bmi with the ba sfo getting first dibs. Once some balancing takes place there will presumably be more opportunities for ba sfos that were comparatively not there before. What there won't be is an instant reduction of the time to command by X(arbitrary no) years, as some seem to expect.
Also from the latest iag forecast of ba airbus hulls this is increasing over and above the current number with potentially the iberia order being diverted to ba as they shrink iberia down.