By the way, physical books are vat-free in the UK (presumably because it is seen as a good thing to encourage people to read), whereas e-books have always been standard VAT in the UK, making e-books less competitive.
The only question is about e-books, which are sent to UK customers by a USA company from datacenters most likely in Ireland or Germany. You could argue that UK VAT should be paid whenever an UK customer buys a digital item from abroad, but it's a bit different argument than your post is making now.
This applies to websites and shops. Europeans will often be surprised in American shops that the tax is added on afterwards — I've had the exact change ready and been caught by this.
The line before "Total" on a receipt shows the amount of VAT paid. Many shops will "*" any items had different rates (things like basic food).
I'm rather sure that it's mandatory. The thing that sucks is that the VAT-included price is visible only at the end of checkout, which is different than everything else - local stores and web-stores are required to advertise only tax-included prices. Possibly it's because the "same customer" may get different tax rates depending on where I choose to ship the item.