What the above allude to is an age slant on the seniority list. There are lots of people like myself in the 35-44 bracket that are gonna sit in the top 1500 places for the whole of your time in BA. So factor that in.
The other slant you may wish to consider, is to apply to start from LGW on the 737. Being junior under carmen is not half as demoralising as having NO control over your life. The flip side is you will work harder than LHR...
Myself, started LGW years ago, currently LHR skipper on the Airbus and flying a leisurely 600 hours/year. Some on our fleet get LOTS more than that, but you can chill if you want.