For all shoppers ordering to outside of the EU and the UK, the tax will be automatically taken off at the last step of the checkout, when you're selecting the payment option. This means that you will still see the total amount with tax in the shopping cart, as the calculation for tax-free shopping is done later in the checkout process.
There's no extra line indicating "-21% tax" before the final order amount, so it may look like the tax is not being taken off. But rest assured, when you're comparing the total price in the shopping cart to the total price before payment, you will see that it is indeed taken off!
Any other discounts are calculated based on the tax-free price, not the tax-inclusive price listed on the website.
It may also look like at first glance like it isn't actually 21%. The normal calculation of "Price minus 21% = Tax-free Price" doesn't work and is incorrect. This is because the tax is added on top of the tax-free price to form the normal price (Tax-free Price plus 21% = Price). If you take the product price that is listed in the final overview, or your invoice, and add 21% to it, you will see that this is the same as the price listed on the website!