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

I need to send a prestamped envelope to my former professor so that she can mail a reference letter for me. She resides in the UK, and the letter would need to travel from the UK back to the US. I, however, live in the US. So, my question is: can I just put US postage stamps on the envelope I send her or do I need UK postage stamps? If I need UK postage stamps, how would I go about getting them here in the US?