Reward Flight Upgrade

Hi,
I am flying NRT-LHR on a reward flight using my miles in PE. I'd love to fly UC back, is there any point asking at the airport for an on the day upgrade or can it only be done by using more miles? I would consider paying some money towards it because I don't have enough miles unfortunately to do it
Flying UC would be a great end to a special celebratory holiday
I am flying NRT-LHR on a reward flight using my miles in PE. I'd love to fly UC back, is there any point asking at the airport for an on the day upgrade or can it only be done by using more miles? I would consider paying some money towards it because I don't have enough miles unfortunately to do it

Flying UC would be a great end to a special celebratory holiday