Originally Posted by
guv1976
Actually, it's just the reverse: Taxes, properly so-called, are much lower departing the U.S. compared to departing the U.K It's in the carrier surcharges where the discrepancy you are talking about occurs.
Edited to add: And the OP is not even departing from the U.K.
I used the word taxes because that’s the word the OP used. Many use taxes when they mean carrier surcharges and using the same terminology as the OP helps to provide a simple answer to a simple question. But call it taxes, carrier surcharges, YQ fuel charges whatever you want, the point stands Avios flights departing from the US have higher add ons than those departing the UK or EU. Departing from Dublin versus the UK makes no difference to this basic fact. Booking an open jaw from the EU/UK to the USA will cost more than a round trip ex EU/UK.
Perhaps you have a better explanation for the OP?