I use Mway services all the time and have never paid. Just park up with the general population and sleep. If you use the dedicated caravan slots be prepared to be woken by a security officer asking for payment.
We stopped in a motorway services near Taunton recently, what a nightmare.
We had 2 4x4s with caravans attached on one side who had stopped for a cuppa and a snack who dumped all their rubbish outside and drove off, I'm sure people thought we had done it. On the other side were 2 HGV's, both drivers used the truck wheels as toilets then carried on a conversation if you could call it that F'in and Blindin at the top of their voices. We had enough then and packed up and drove on Motorway Services no thanks I'll end up in a brawl.