BA318
The actual answer is it costs very little to have a split operation. Handling is outsourced and will be done on a per flight basis (or in the case of Ryanair, on a per passenger basis) so this is negligible. As for facilities - this is the UK, not the USA, no overseas airline owns or leases any facilities here, bar a few lounges at LHR. The only real additional expense would be engineering provision in two locations. Currently Boeing handle engineering for WS at LGW. One would assume they’d need to source a different company at LHR, but this is another very small expense in the grand scheme.
Very few airlines split their operations because very few airlines need to. Once they get enough slots at LHR, there’s no point staying. But the majority of airlines with a sizeable LON operation that migrated from LGW to LHR started with a split operation until they had enough suitable slots. There’s a difference between a split operation to capture different parts of a large market, and a split operation until suitable LHR slots can be obtained for all flights.