As Yellow Pen has already stated, BA do not use LHR as a destination and alternate. Nice weather and all things being equal, LGW would normally be the filed alternate. On the day, running out of petroleum and thrashing round the LAM hold, a lot of guys will re-designate STN as their alternate and calculate their fuel accordingly.
However, just occasionally, when the weather's nice and within 6 hours etc etc, LGW might get thrown away and the fuel that gets 'freed up' is then used to increase the holding time available, if required. This is clearly not planned, so doesn't contradict the original answer of 'no' but having the 2 separate runways is sometimes used, as per the rules.