So there are roughly 9 departure slots between 0700-0900 , 9 arrivals 1300-1500, second wave of 9 departures 1400-1700 arriving back 2100-2300.
There are very few early morning arrivals at LGW and very few mid morning departure slots.
But there are plenty of early morning arrival slots available at Gatwick. LGW is only severely slot restricted at peak times for based aircraft (early morning departures, late evening arrivals).
If BA want to use the slots to expand long haul, the biggest problem they will face will be gate space.