Going through a wild back-and-forth with agents at BA over what I think are absurd and (possible illegal) taxes for a one way UuA from Club to First.
Route is BOM-LHR. For the upgrade they are asking for 22,500 Avios which is correct. But the mess begins with the quotes they give me for the added TFC.
First agent quoted ~260 GBP. HUACA.
Second agent quoted me 800 GBP!!! HUACA.
Online upgrade quotes me ~300 GBP
Let's ignore the inconsistencies (do they expect the customer to just go with the lowest quote!?).

I did some digging and I think they are doing something very odd with the TFL. So the Indian version of the VAT is 18% on premium travel. So you pay 18% of (fare + YQ) and its called the Goods and Service Tax.
For the original ticket (return in J) the tax is 135 GBP. For the upgraded ticket (outbound F inbound J) the tax jumps to 380 GBP because they are calculating 18% of a 2,100 GBP revenue J+F fare! But why on earth would I pay VAT on a cash component that I have not paid the airline? I am not paying the airline 2,100 GBP. I'm paying 22,500 Avios! The increase in YQ between J and F is marginal (~20 GBP) and I should only pay VAT on that increase. How is this kosher taxation practice? How does BA even plan to file that with the government? The Indian tax regime does not expect you to pay VAT on a payment you DONT make to a merchant. Makes me wonder if they have been making consumers pay for a tax that they do not actually send to the government. They simply pocket it?