(A). Yes, you can do this. You would be able to call later to change your destination, which should be no problem (subject to availability for the new destination final leg). You would be charged change and offline booking fees.

(B). You can also do it this way. You would be able to call up and add your final leg to the existing Sfo(?)-LHR booking, without losing the original booking. Again, you'd be liable for any extra taxes, avios, change and offline fees. This is the best way to secure your long haul legs when they are available.

Finally, book quickly, those west coast long haul seats disappear very quickly sorry.....