Wirelessly posted (BlackBerry: BlackBerry8530/5.0.0.1030 Profile/MIDP-2.1 Configuration/CLDC-1.1 VendorID/417)
"So I could do a one way into LHR using Avios and there wouldn't be any taxes?"
The "taxes" -- properly so-called -- would be minimal. But if you redeemed Avios for TATL flights on AA or BA metal, the "surcharges" would be enormous.