Hiya there, as someone who is currently at LGW (Longhaul) I can tell you that if you went to LGW Shorthaul you will go onto the Single Fleet when it comes around. BA are starting to shift the current Longhaul people who want to move to LHR in October this year and are due to finish next year Feb. So even though you are entering as Shorthaul you will end up doing both. As for LHR vs LGW there are no guarantees .... if BA hire you then you go where they can fit you in and not necessarily where you want to go. They will of course attempt to satisfy your requests but at the end of the day operational needs will always come first at the airline. Hope this helps .... any other questions let me know. Good luck!!!