I've only used the tap to pay once to test it with my own card and once with Mrs. Catcher's card. On my phone you need to tap the rear camera with the card. However, I find the card reader more convenient in the car so it's only really a backup facility in my case... at least until I break or lose the card reader.
You can remove the VAT by logging in to your Sumup account through a www browser. I can't remember the exact procedure but it's straightforward enough, IIRC someone mentioned it on here before. You can also update the receipt details to remove your address if you wish.