Unless the supermarkets agree to notify the website, I don’t see how it can work well?
I’m fairly sure the supermarket sites have ‘bot detection, if that’s what it is called, because if I just keep trying, over and over again, it never works, I never get a slot, as if my ip address has been sent to back of queue. The occasions on which I have been successful is when I pick up the tablet after a few hours, out of the blue, and then, if I’m very lucky, I get offered a slot on the first attempt.
The other issue they’d face is timing and response, you’d need to be pdq quick, especially if racing against other users. I’ve noticed, when I occasionally check as above, and find new slots have appeared... even if I don’t book one, if I wait about 30 seconds and look again, chances are most if not all slots will have evaporated.