From the Global Settings (Home -> Members Area -> Selling -> Seller Tools -> Global Settings) menu select to:
Enable Tax on Listings
If checked, allows sellers to apply tax on listings.
Select the tax that will be applied to listings.
Note: The tax is 20% VAT applied to residents of the UK.
The seller has the possibility to Apply Tax on a per listing basis from the Create Listing menu:
Log in and the software will automatically apply taxes on products depending on the country of residence (billing address)
The invoice will display the final price including tax: