This is fairly straightforward.

2 for 1 Voucher, BA Metal:
2 people = X miles and about $650 per person for the simple trip to Europe

Single OW Partner:
2 people = 2X miles and a fraction of cost in taxes per person for the same trip.

So for US-LHR/Europe 1 in F, you either pay about $1300 or an extra 150K BA miles. I personally choose the BA metal option since it's almost always my wife and I traveling together.

Now if you wanted stopovers elsewhere from LHR, to fly somewhere besides Europe or Africa where BA isn't an option (or at least shouldn't be based on the mileage cost) or between regions not including US or LHR, that's a separate issue.