BA has really cut only on SYD and cut MEL. Why could HNL over these destinations since the traffic would be lower.
BA needs premium traffic; ie business travellers - so HNL will not qualify.
BA like most other airlines have realised that position crew for a two sector haul (LHR-SIN-SYD or LHR-LAX-HNL) is expensive with a minimal onward load LAX-HNL or SIN-SYD) since they are a third party carrier.( Would you fly Air India to JFK which does BOM-LHR-JFK?
In any case, I for one believe Hawaii is very over hyped and is terribly expensive for what you get. You can the same luxury holiday with tropical climate at a fraction of the cost and with much better service in South East Asia compared with Hawaii. Been there, done that and no inclination to repeat the experience.