A few days ago I redeemed some Avios for a BA ticket from here to London in December, and most of the taxes were UK taxes, but I agree with you LHR is terrible in that regard.
Taxes in Argentina are generally very high, and the air industry is not an exception. And under the current economic climate we shouldn’t expect any changes. That said, foreigners get zero VAT in hotels when paying with credit card, which can be a lot of money.