BAreject
You are under no obligation to return. You could reject the offer of a job at LGW and wait to see if one becomes available at LHR.
If it was not for the BACC's efforts, there would not be any opportunities for you at LGW - they would have been lost to BA pilots for ever.
If you don't want to go to LGW, there will be many who will be happy to take your place and many others who would do it for free. Count yourself lucky to have the option.