I will add another data point/comment.
Upgraded a UK paid Club seat to First for my return flight from Boston in July - 20.000 Avios as expected and £124.30 'tax' - when I questioned this as I thought J and F YQ's were the same, I was told 'no, there is an increased tax fee and it actually changes daily'
I know that it is actually a fee not tax, but was surprised that it changes daily - I assume based on USD currency fluctuations.