JER - LGW vs LHR
Think the point is opportunity cost for BA and (lack of) potential loss of business.
Given no-one else offers, JER - LHR, BA aren't losing out on pax using that route offered by someone else.
JER pax wishing to conncet long-haul are stuck with LGW and now pretty much stuck with BA. Therefore, whilst nice of BA to consider convenience of straight connections at LHR, they aren't missing any traffic (bar those going to MAN or other places to connect) so no real incentive to go for a routing that would probably be more expensive and takes away precious slots.