That is good for a day or two, however I would not expect you will have a satisfactory resolution within that timeframe. Some reason BA calculates that tax in a certain way and as this is a tax question even rev management can't overrule it. The only one quick solution would be to making an exception and creating a special fare for your case but that won't be valid for next time and nowadays BA is not really wants to go down in that rabbit hole. If they are miscalculating the tax, that would take weeks and months between various departments to amend the calculations, hence my suggestion to pay now and argue later.