Can I use US postage stamps to mail something from the UK?

I don’t think so. I think that all the international mailing that I’ve done either used lots of domestic stamps or else a sort of international stamp. It’s been a long time.

I would go to the British Postal web site and ask them.

Yes. Here’s a link to the USPS page with information on mailing internationally.

No. Your postage stamp pays the U.S. postal service for handling your letter. The British postal service is the one that wants to get paid.

There is, or used to be, such a thing as an “international reply coupon” for prepaying postage across international borders.

I was wrong.

They won’t accept a fax or pdf through email?

What you need to purchase is an IRC (International Reply Coupon) from your local post office and include it with the return envelope. That will cover the return postage.

@AstroChuck : You beat me to it. You’re exactly right.

I mentioned the IRC above.

@Jeruba : I see that you did. Kuddos to you, too, for beating me to the punch.

Well, he is a professional.

No, just as a UK stamp won’t work in the US.

@AstroChuck Nearing on 30000… Wow!

The short answer is “no.”

The UPU establishes the conditions on the exchange of mail between countries. Each country receives revenue from the postage on out bound mail, so postage has to be paid in the country of origination. You need to send an IRC.
Countries that are net exporters of mail compensate those that are net importers. The US is still a net exporter of mail.