Sold two cels at once, so when I mailed the first 
(domestic) one, I asked the clerk at the post 
office about it.  Told me cheapest way (and this 
is assuming a cel or two, stuck between a good 
thick cardboard layer) was to get one of the 
Global Priority Mail envelopes and do that.  
Worked great--got there in three days and set me 
(well, the buyer) back $10.

One big thing to watch for when shipping to UK: 
ALWAYS mark the customs form as "gift/personal 
papers, $0 value" so that your recipient is not 
hit for customs charges.  Let the recipient know 
you're going to do this, too, so you're covered on 
all sides.