In order to get the EIN number you need to get form W-8. I didn't have to send the form in, I just phoned this number (00)1-267-941-1099 the woman asked me the questions that were on the form, then gave me the EIN number. While on the phone I made it clear the I could speak on behalf of the business. A week or two later they sent confirmation out to my business address. I was able to use the EIN number immediately.
The guys on the end of the phone obviously deal with people from the UK phoning up about this number all the time - so they can easily guide you through it all.
When filling the form in, there's a PDF of instructions for filling it in here: http://www.irs.gov/pub/irs-pdf/iss4.pdf which I missed initially! The W8 form itself is available here: http://www.irs.gov/pub/irs-pdf/fw8ben.pdf
I found this thread quite helpful when I was researching how everything worked: https://www.createspace.com/en/community/thread/17723