You can, just use a US address from a state where you wouldn't incur tax for the transaction (alaska for example). All you get is a CD Key (in my experience), so grab the key, if it's steam you don't really need the amazon service any more till you buy something else.
The Amazon.com / UK customer issue gets brought up and talked through so often in this thread, any chance we can put details in the first post, or is there a comprehensive post we can link to as to avoid bloating the thread with run throughs / dangers and so forth?
I'll have a look when I get a sec if no one else has I'll try to collate the information here and online so we can refer back to it when it comes up again.
EDIT: Two Fiddy! Yeaah!