Dunno how you'd get the shortcode approved in US if it's hardcore - even if you're not sending hardcore, if the website backing the SMS campaign is you'd still have issues. And I'm not sure, but you may need separate shortcodes: one for the US and one for the UK, though I could be wrong on that.
However for softcore stuff, we're using SimpleWire as our SMS aggregator -
http://www.simplewire.com/ (now Open Market).