I fly as a customer form both LHR and LGW with BA and would say the level of on board customer service on flights out of LGW is exactly the same as it is out of LHR., call bells are answered immediately and there is no delay in the delivery of the service or the collecting in afterwards.
One thing that is noticeable is on the short haul LGW flights, they have far fewer cabin crew than they do on same aircraft operating out of LHR (A 319's) when the cabin service is identical.