Haven't sent a whole frame from the UK, but when Kev sent me those rims we used Transglobal Express (http://www.transglobalexpress.co.uk/), they were waaaaaay cheaper than anybody else. Might be worth a look.
I sent a complete Kona Cindercone to andrewl in Oz with parcel2go for £102, assume frame might be cheaper - though really it's the volume not the weight that's the issue.