If you're in the UK and registering with your Limited company, you need to put your company name in the "Tax Identification Number" field, and your VAT number in the "Registration Number" field (if you're registered for VAT, obviously.)
If you're not sure what your VAT number is internationally, you can check it here: http://ec.europa.eu/taxation_customs/vies/vieshome.do?select...
For the UK, you have to put "GB " in front of your VAT number.