Hey Rico, without bogging you down with the complex organisation, if you join EF LGW chances are you WILL be flying on the 777 within the next twelve months.
Now for the science bit lol: Although EF LGW forms part of 'mainline' they do not form part of the BA Cabin Crew National Sectional Panel ('NSP'). What this basically means is that the LGW EF crews work to completelly different Terms & Conditions than EF LHR/MAN & GLA and WW LGW/LHR. Things that BASSA have negotiated for EF LHR such as meal allowences, minimum turnaround times and much more do not apply to the EF LGW crews.
Perhaps most importantly though, EF LGW not being part of the 'NSP' means that they cannot transfer anywhere else within the BA network. For example, if you are EF LHR you can put your name on the 'transfer list' for any other NSP base or fleet. So someone joining EF LHR can transfer directly to WW LHR or someone joining WW LGW could transfer to say EF MAN. There IS a 'gentlemens agreement' between BA and the unions which allow a certain number of EF LGW crews to transfer to EF LHR, but that is it.
What BA is aiming to do (to cut costs) is to 'merge' the 9 WW LGW routes into EF LGW and relocate the exisiting WW LGW NSP crew into LHR. Therefore, EF LGW crews would also operate the 777 longhaul flights as well as the exisiting shorthaul ones. Albeit, the longhaul flights will be operated under 'new' terms & conditions and payments (ie less) than the current WW LGW NSP crew.
Now if that has absolutely confused and exhausted you, then you will be all set for a life in BA! Lol