The LGW 'future fleet' has been under discussion for a long, long, long time with the decision being pushed ever further back.
The 737 is on its knees (hence doing a Glasgow Shuttle in the 777!) and need to be replaced. The question, as always, is how much cost against profit can the SH model at LGW bring in the face of Lo Cost competition and that decision is well above my pay grade. Perhaps potential future IAG acquisitions may well add to the picture if you get my drift.
As far as the new 777-300 hulls are concerned,
AFAIK, the crewing has already been forecast in the current manpower availability plan. The 787 pilots are converting through the 777 before the 787 aircraft gets delivered hence the extra hulls are covered.