Possibly an innovation in Manchester, but WGs have been plying their trade in them for years in Europe.
I well remember passing quite a few parked up off the Viareggio-Pisa superstrada some years ago. The 'available' ladies were sitting next to their respective campers, reading patiently, waiting for punters.
All very civilised, I thought. Only us Brits could call it a 'fuck bus'!