I thought I'd share this, since I've been dreading having to apply for my ITIN (Individual Taxpayer Identification Number) and then some nice person pointed out that if I am an indie author publishing my own books in the UK (and other nations), I can actually apply as a sole trader for an EIN (Employer Identification Number), rather than an ITIN, which is a whole lot easier than applying for an ITIN.
Taking a step back to explain this - if you are getting paid by Amazon, Smashwords, Google Play or anyone else based in the US and you are not in the UK, you will get slapped for 30% tax from the US IF you don't give your 'payer', i.e. Amazon, a completed W-8BEN form with an ITIN on it or an EIN (link to w-8ben and instructions for w-8ben).
I found two very useful resources online:
This has an example form for amazon (ignore the ITIN, use EIN)
this gives you a step by step guide to filling the 8W form in for Amazon, Smashwords and Createspace.
The second set of info gives you an extra bit of the form to fill in, because, the withholding percentage is different for different types of royalties, which is listed in the instructions. The first example direct from Amazon does not list this, but (and this is my supposition, so don't take my word for it) it might apply if you wanted to spread your tax payments for royalties over more than one year, which authors are allowed to do in the UK, rather than just looking on them as standard self employment income.
Registering as self-employed in the UK is really easy, just go online to hmrc.gov.uk and follow the links for registering.
I had my EIN within 10 minutes and I'm currently filling in all my forms. I will update this post if I have any follow up problems.
EDIT: having been through this with Amazon and just got my 0% withholding, this is the best example form to copy for them, just use your EIN, rather than an ITIN: W-8BEN for an individual Amazon