I have been looking at CW flights for SFO-LHR. I also looked at upgrading with miles to First and found a one way upgrade which costs 50000 miles and also 1200 pounds more than the CW return flight.

My question is why is there a 1200 price difference, I thought the 50000 miles is supposed to cover the one way upgrade to First?

Can I book the CW return flight and call up the EC to get the one way upgrade with miles?