What will happen depends on several things.
If you are well endowed they will be envious and may arrest you for exposure.
If your girl is well endowed they may insist on taking down her details, and allow her to get dressed later.
My personal thinking is that they won't give a f**k and will drive-on to another place.
If you have a gang of guys standing by the side of your motor wanking, all bets are off. In "the old days" any dogger who stood about wanking while a car was driving in would have been sent to coventry for idiocy.....sadly, idiocy is not in short supply anymore.
What Mr G is saying, in the succinct way he has made all his own, is you should do a search of the site.....
From very personal experience of being caught four times in as many weeks semi naked in the back of the car with my partner, I can confirm that the only thing Plod seems to be interested in is 'is everthing ok sir ,madam', this always seems to be asked politely like a waiter enquiring over your meal, then with a swift flash of his maglite over the flesh they are gone........even had a WPC peering in last week !!!!
As for the 'New Doggers' they are more at risk but most of them have all gone to Bedfordshire as Jomu will confirm.