There are different booking class's on BMI redemptions? or did you change cabin? that may require a tax change which I presume would need to be re-ticketed.
tht
It was cabin's change (C-->F) : taxes were the same as fuel surcharge is always the same and as route's origine wasn't UK (APD tax). Only more miles and cash (and 20£ change fee + 4£ credit card fee).