"Basic new starter pay at LHR is c£11400.
Basic new starter pay at LGW is c£10500."
So, what are your contracted hours per week?
If I assume (waiting to be corrected) 37.5 hours per week, then on the minimum, this wage equates to £11203.58 (if over the age of 22).
18 - 21 @ £9209.23.
National Minimum Wage - BERR
If the company are offering a staring pay of £10500 at LGW and you are over 22, then the company is breaking the law!