In USA prepaid cards can be found at just about any major retailer, but they're also available online. I believe gospendit prepaid cards work in the UK but I recommend you wait on the input of a UK member for specific details.
As you're shopping around keep in mind you want a "one-time prepaid gift card" rather than a reloadable card. The reason being is reloadable cards have to be registered in your real information while prepaid gift cards do not have that requirement.