Cottaging, meeting for sex in a public toilet, has gone on the back burner for prosecutions. The only way to get a sure nick is to lure someone in on a promise. The Colin Stagg fiasco ended that.
Dogging, watching someone with their consent, is actually legal...see the sex offences act 2003. Whether the couple are breaking any laws by having sex in a public place is open for discussion, since the police can now prosecute on their own sight...
Dogging, watching someone having sex or watching someone with various parts of their
body exposed without their knowledge or consent, for sexual gratification, is now a serious sexual offence.
Exposure, standing by the side of a car and wanking, is now also a serious sexual offence.