LHR-SFO
SFO-MEL
MEL-LAX
LAX-LHR
Try searching like that?
Its too late to see if it will come up/price the same as what you came up with though, but this should give you a good start.
You may also want to try a 3rd party site, Orbitz/Travelocity/Priceline, they might be able to add 9 multi-city segments.