Giff Gaff micro sims are not sent out by Giff Gaff but rather the Giff Gaff community having been chopped down with a SIM cutter.

I'd be inclined to approach said community directly via the Giff Gaff forums and politely asking if one of them would be interested in shipping to the USA in exchange for you sending them the postage via PayPal. Surely it is worthwhile to cross their palms handsomely with silver in order for you to be able to hit the ground running.

Say if I were to go to Miami I would be paying $10 plus tax for a T Mobile SIM then $3 per day for pretend 3G coverage. I am sure if you were prepared to offer that same $10 or maybe even £10 then a GiffGaffer that is on the way to the post office anyway wouldn't mind shipping you one.