Ah, OK.

Well, basically, anywhere is cheaper than the UK.

Whether its cheaper duty-paid or duty-free in the US probably depends which state you're in, so I'll leave that to one of the USAian posters to answer. In Amsterdam, duty paid is €39, so, if it's cheaper than that in the US, go for it. Flying from the US and Canada, though, I've actually found in-flight duty-free often to be cheaper than at the airport, though.

Apologies, can't be much more precise than that...