I agree with everyone else. I am getting my mail forwarded from USPS.
As for filing in the Boxes... I just wrote ENGLAND or GREAT BRITAIN in big letters after the post code. The yellow forwarding strip is on them, and I've been getting my mail.
If you are moving a bit afterwards, I would just grab a couple extra forms and bring them to the UK with you. When you know your new address, just send them off to the US Post Office, and I bet you'll get your new mail forwarded too. And there isn't a charge.
Of course, Royal Mail also forwards your mail when you move house, so you should probably fill out one of those forms when you get your new place too.
Lisa