Thanks for getting back to me, Mee1977.

I've already updated the screenshot to be specific for UK region users. 

The Charge Sales Tax box is only available for US region users.

Assuming that you've already set up VAT, you should be able to enter it after following the steps I provided earlier.

If you still need to set up VAT, you can use this article for reference: Set Up and Edit VAT Settings, VAT Codes and VAT Rates.

Let me know if you need anything else.