Oh dear! We didn't mean to presume, and I'm very glad you brought this to our attention.
My understanding was that the ASFA website indicated the Chesleys are awarded at the NASFiC when the Worldcon is not in North America, but I'm guessing that information was taken from http://www.asfa-art.org/pages/05-chesleyawardspage.html
, which only says "The awards are usually presented annually at the World Science Fiction Convention or at the North American Science Fiction Convention when the Worldcon is held outside of North America."
Usually certainly does not mean definitely, and I have amended my post to reflect your comments.
It was also mentioned to us by someone from the 2010 NASFiC in Raleigh, and reflected in their budget, which we were given some numbers from, to use for reference.
It's important for us know that the awards might not be at the NASFiC, as it changes what we might say to the hotel about potential banquet expenses. Thank you very much for the news.
Is ASFA considering having it in London, then?