You are wrong. You can take a cancelation if you've already got a test booked, I'm unsure if you can just book a canceled test online without already having one booked. I booked a test for a few weeks away online when I got home from my theory test then though "f**k it, I'm not waiting that long." And looked for a cancelation, one came up for 11 the next morning in Winchester and I booked it. Went and sat through a bit of theory with the guy I'd booked my driving course with and then went for it.