I had a nightmare trying to book something similar for my girlfriend - WT+ out and J back AMS-LHR-SFO. The guy at BA was extremely snotty when he was unable to get the price I was after. He was several hundred £'s off the web price for WT+ return which seemed to be the problem.

In the end I did what would have made more sense in the first place for me - I phoned my local (Hong Kong) office where the dude spoke great English and there was no booking fee. He hit the right fare first time.

There are two theories - the UK systems are muck and (more likely) UK agents very rarely have to book complicated non-ex-UK itineraries so they get caght out by HIP and other such weird routing rules when they don't enter the fare correctly.

Good luck!