Figures for LGW for years 1-5 (your engagement freeze) will be pretty much the same as LHR as the pay points are identical and the allowances are pretty similar. There are less nightstops and therefore duty pay will be a little less but the figures for SH will be similar. If you work on the LHR figures then take £200 per month off I doubt you would be disappointed. The issue is that unless the few £ per month difference is a deal breaker for you, I would not wait for LHR if you can get in earlier and get your seniority number sooner.