Unmarked police car doing speed checks

On the way home from work today there was a unmarked police car parked at the side of the road doing speed checks, I could see the speed speed gun as I got close. I'm wondering if they can issue speeding fines like the vans or do they generally pull you over if they intend to prosecute. I was slowing down as I came into the 40 but not 100% convinced I made it down below the speed limit by the time he would of seen me.