Sniping with AS is just like bidding directly on eBay: your bid is in the same currency the auction is in. If the auction you're interested in is in pounds, then your snipe would be for 100 pounds, not 100 dollars. If the auction was in euros, your snipe would be in euros.
Your bid goes in whatever the currency of the auction is. If the auction is listed as dollars, then 100 is 100 dollars. If the auction is listed in pounds, then your bid goes in as 100 pounds. If the auction is listed in cruzeros, then your bid goes in as 100 cruzeros, and so forth.
Beware that the Royal Mail is one of the most expensive postal services in the world for international mail now, so be sure to factor quite a big chunk in for US-->UK postage into your bid.
I'm looking at an item that has shipping to the US listed in the usual spot as 15 GBP. Someone asked the seller a question as follows...(I don't know why he asked when it is shown...)
Q: Do you have any idea how much shipping would cost to the U.S.? Thanks Oct-08-08
A: 29 pounds 5-8 days, and 44 pounds 4 days insured. I know, it is very expensive , but that is the online check prices for the post.
So he lists 15GBP and answers a Q. as 29GBP. Thats a 14BPB difference = $24. 15GBP=$25. I checked Royal Mail online and they listed 3.5lbs at 12GPB international. I'm inclined to bid and if I win hold him to the shipping price he has listed on his auction. If he can't ship a camera flash unit for $25 he's full of patootie. What do you think about that?
