Finally after a lot of calls with BA and a very helpful member of the GGL night shift team in NCL I was able to get all of this sorted.

Itinerary is now LAS - PHX - JFK - ORY - LHR - NCL in First / Business including Open Skies (so interested to see what the Biz Bed experience will be like) for $3 more than the direct LHR routing and 40 extra tier points.

Return is unfortunately just in WT+ direct from LHR to LAS. My budget only stretches so far, and it's a daytime flight so not too fussed. Hopefully I can UUA closer to the return date.

Thanks again to all who commented and gave suggestions as to how to get this booked. FT at its best