What's the correct country code/prefix for a UK unique taxpayer reference I should give in an invoice when supplying services overseas? I suppose it's simply UK, but I couldn't find anything about it on the Net. I found some articles regarding tax numbers in other countries, and they for example say that in Poland you should add PL before a tax number, that's why I suppose it'll be "UK" in the UK, so:
UK 1234567890Am I right?
I think you're mixing up the Unique Taxpayer Reference (UTR) and VAT registration numbers.
A Unique Taxpayer Reference (UTR) is a number given to individuals and companies and is a way for the tax people (HMRC) to idenitify you.
No one apart from you, your accountant and HMRC should ever need your UTR's unless you're in the construction industry which has a special scheme (CIS) A VAT registration number has different formats in different countries and is prefixed with the country code UK, PL etc. You should show your VAT number on invoices (assuming you're VAT registered).
You can find all the formats for VAT codes issued by EU countries here and you can verify VAT numbers here.