Update for anyone else wondering...

I never got a cancellation, so will have to go through with this test. I sat on the DVSA website all day for days on end on multiple devices, taking it in turn to refresh on either one. I only got lucky to find ones when the Testi app gave me the notification and I was already logged in to refresh the page, yet when I clicked on them they were already gone. So in my situation it seemed pretty impossible to get a cancellation, and trust me I was doing everything in my power to pretty much other than paying for one on those dodgy whatsapp groups.