WW LGW and LGW EF are to be come one fleet.....ie mixed flying in a few years time. However, T&C, pay etc... won`t be on the current WW LGW agreement. It will be on the LGW EF agreement more than likely. So really you would be better of going to LHR EF for a better life style pay T&C. The frequency between MRS and LGW aren`t the best either for commuting and if the flights full you won`t be on it. NCE-LHR is better frequency however, flights tend to be busy but other BA staff with high seniority will be grapping the available seats. BA also have a habbit swapping routes between LHR and LGW.
Think you may have a little more to thing about than a MRS-LGW commuting route for you decision for joining BA.
Good Luck!