Mailing birthday card in envelope to UK - cost?

Current cost to mail a modestly-sized, rectangular birthday card to the UK is $1.10 (April 2013).

Note that as of January, 2013, the Global Forever stamp can be used to mail a postcard or 1 oz letter-size mailpiece anywhere in the world, regardless of when the stamp was purchased and how prices change in the future. To help you distinguish the difference between the domestic and international forever stamps, the international version will always be round and will always bear the Words "Global Forever." Right now, April 2013, these cost $1.10 each.

Also note that the postage goes up to $1.30 if the envelope is square or an "odd" shape. This web page might be helpful:

http://ircalc.usps.com/?country=10143