American Airlines is offering a 50% bonus when you buy miles. The deal is valid until 31st December.
I am not an expert on buying AA miles, but View from the Wing rates this as about as good a deal as you ever see for buying AAdvantage miles.
The ink to buy AA miles is here (click on the AA logo).
The best value comes when you buy 60,000 miles (the annual cap for buying AA miles) to receive 30,000 for free. That is 90,000 miles for $1,685 including the fee, or 1.17p each.
Even taking into account AA’s low taxes (business class London to New York is £177 of taxes compared to £520 with BA), it still isn’t worth buying all the miles you need for a redemption at this price. It is not a bad price if you need to top-off an account, though.
The ‘sweet spots’ for AA redemptions for a UK resident are the Middle East and India, where business class (flying on a BA plane) is 60,000 miles return vs 80,000 using Avios. First Class is 80,000 AA miles versus 120,000 Avios. You still pay the same taxes as BA would charge, though.
AA is also partners with Etihad which offers you an alternative route to the region via Abu Dhabi. I don’t know what the taxes are for London to Abu Dhabi on Etihad, using AA miles, but I imagine they are lower than BA and the onboard product in Business and First is meant to be excellent. It is only a short drive from Abu Dhabi airport to Dubai as well.