With LAS, I think it was a case that they were getting a good feed out of LHR, and with the LGW it was just too much capacity - so LHR stayed.
You're quite right in saying there are still options out of LGW for BA if they want to explore them. Unlinking AUH & MCT, and moving MCT to LGW could be an interesting option, along with a much talked about SEZ (definitely 2x week would work on that).
Along with Norwegian, you also have Thomson - they are now flying to Phuket, Colombo, Mauritius - along with current BA routes like Orlando, Cancun & Punta Cana. With once a week rotations it would be easier for them to fill the aircraft though.