I've ridden around rivington many many times at all times of day and night and only once found a young couple in a car, not exactly what i'd consider "dogging". I also ride around Mugdock and have never seen any such action here, but I suppose that's probably because I'm there at odd times.
Ainsdale beach (sefton coast) seems fairly popular though, and actually quite intimidatingly, so much so I resent it. I'm all for people enjoying themselves given a quiet moment wherever they like, but on a number of occasions I've turned up at the beach to try to get a couple of hours in (kitesurfing) as the sun sets and found cars "moving in" as I get changed into my wetsuit, then people appearing near the car. And on more than one occasion the missus and I have gone for a bit of a sunbath in the edge of the dunes and spotted blokes in the long grasses beating one out – nice, real nice 🙄
I used to be fairly liberal about such things but as I get older I find it more disturbing and more unpleasant.