You'll have to go to the US authourities and request your police report from them directly.
Go to your local UK polioce station and request the form (the one I PM'ed you about) and send in the £10 with it.
Then separately contact the US authorities to get your report from the US. I don't know how much that will cost, or who you go to (I'd have to do a bit of research about that, and I'm inbetween a feed and a diaper change at the moment!). But the US one (or anywhere overseas you've lived for 6 months or more) is a separate process. Whetever form they give you, bring it to your interview along with what you get in the mail from the UK guys.
Hope that helps - when I get a minute I'll try to find out how to get the US records, unless someone on here knows?
Added:
http://www.usembassy.org.uk/cons_web/faqs/iv/faq_pc.htmFAQs about police records.
Doesn't say how you get your police records from the US though... Hrmmm. Still, it might be helpful.
*thinking (typing) out loud here*
*goes off to look for more info*
MarsBar, didn't you & your hubby live in the US together before moving to the UK? Did you have to get a US police record for him?