UK VAT Automated Invoices

HI - How do I create UK automatic invoices for people who have bought from me on Amazon..... I tried to register the VAT number but the UNITED KINGDOM was greyed out

If this is not possible on Amazon are there any plug-ins I could use..... I wouldnt be able to upload the number of invoices required manually.

Hope someone can help as Amazon support has been awful on this topic.