1. Login as admin
2. Open General settings page and activate "When a customer isn't logged in, it is presumed that he is from the default country" option. After you have activated the option all anonymous users (before they log in order enter registration information) will see prices for UK location (with taxes value).
3. Open the VAT 17.5 tax page at
http://nikkoe.com/xcart/admin/taxes.php?taxid=1, tick "Display product price including tax" option and save the changes. Now you customer will see prices including tax rate.