So basically what you're saying is I will need to book the two separately?
But I could purchase the Q fare JFK-LHR and do the 25,000 mile upgrade? I'm not thrilled with having to claim luggage at JFK and then go back through security to meet our international flight, but if I know that ahead of time, I'll just give us a hefty layover.
Now if I have to buy a full Y fare JFK-LHR, then there is no point in using the miles. On that particular route, it is an automatic upgrade to BE.