You can change the way VAT is charged on invoices, so that it is always charged, never charged or only charged to clients in the UK.
Changing the way VAT is charged on invoices can be done at the contact level. If you always want to charge VAT when you invoice a contact, just edit the contact and ensure the Charge VAT drop down in the Invoicing Options section is set to 'Always':
And put in the VAT registration number for that contact.
This will ensure that the VAT number is added to the outgoing invoice.