I thought I was in a time-warp when I saw this thread title. The "BOB" signs were introduced back in the 80s, and ~ here in the US, at least ~ are rarely seen anymore. When you do spot one of 'em, it's usually on a car that is 20 or more years old.

Are they a new thing in the UK? Or enjoying a revival?

They're frequently seen in this part of Oz, and often on cars in supermarket carparks. The responsible thing to do would be smash one of their windows so that the poor baby doesn't get heatstroke in the closed vehicle.