"Slippery slope" is a common argument but usually flawed. In this case, driving is an extraordinarily regulated privilege and despite that, it still results in massive deaths and permanent life changing injury every year. In the US, car crashes are the number one cause of death for children. It's difficult to draw a line between expanding driving enforcement to gross losses in privacy like many here are envisioning.

It also ignores the benefits to civil rights. Again, I don't know about the UK but in the US, traffic enforcement by police is very unevenly applied. Minorities routinely get their privacy violated on pretexts while cops don't even pay lip service to the rules.