All of the information about this scheme is right there in front of you.
I do endless amounts of hours of research on this topic as it directly affects me, you'd be shocked to learn the extent the police will go to try and control.
It's all there on official Government websites. They will take the APIS that is gathered up by the airlines and it will be sent directly to the Home Office, this will then be checked against ViSOR and if there is no record of notification they'll arrest you for failure to comply.
Unless you left the shores in a dingy and landed on mainland Europe they will know you have left.