FlyingCo taxes?

I had a LHR->SFO return booked in K. Flew the outbound then got my PA to call FlyingCo about upgrading from K to G on the return leg. Availability there so that's good.
However they said they wanted £250 in taxes / fees difference. Looking at the difference between normal K fees for the return leg ($231.50) and Upper ($454.50) surely this should be nearer £130?
Also the fact that they've actually charged an additional £285 to my credit card, I'm guessing there's some random person blindfolded throwing darts into the 'how much are we going to charge them' dartboard
However they said they wanted £250 in taxes / fees difference. Looking at the difference between normal K fees for the return leg ($231.50) and Upper ($454.50) surely this should be nearer £130?
Also the fact that they've actually charged an additional £285 to my credit card, I'm guessing there's some random person blindfolded throwing darts into the 'how much are we going to charge them' dartboard
