Would dread the thought of having to go LHR-SIN-PER-SIN-LHR-MIA(via wherever)-LHR. Nasty. Very praiseworthy if you do it!

What's the differential between that option and an RTW?

As far as mileage goes you should be able to squeeze LHR-SIN-PER-SIN-LAX-DEN-MIA-IAD-LHR and LHR-SIN-PER-AKL-LAX-DEN-MIA-IAD-LHR into the 29k limit on a *A RTW. You could then bung in a MIA-LAX-SAN-LAX-MIA roundtrip in the middle. That'd add anything up to £250. So maybe just take the 34k *A RTW in which case you could include SAN in the middle.

RTW would be more appealing to me + you'll get yourself a bit of time back crossing the dateline eastbound too (could be a good or a bad thing!).