Jay, Ignore all the hype that you read from a few months ago on this forum regarding appalling take home pay, that was last year when we were going throught the transition to Single Fleet and we had too many crew to man the routes as they were phased over to us.
Now that it is all up and running the take home pay is usually satisfactory, I work at LGWSF and usually take home approx £1450-£1600 per month and I only joined in 2006 so do not have years of annual increments behind me. Of course if you have leave or sickness this figure will be lower.
I have worked at both bases for BA and I have to say that I prefer LGW, it is all down to personal choice but at the outset you work where BA need you, currently LGW, if BA is your hearts desire as an employer then you should take it and transfer to LHR at a later date.