Matt, the ones on the M42 around the NEC work. They use NPR, not flash, for the average speed. However, they also use flash cameras that are on even when the variable speed limit isn't in force!
I've seen a couple of drivers pass me at over 80, boom goes the flash, and their adrenalin goes straight to the brake pedal!!
Personally, I find the variable cameras on the M42 and M25 work well, and keep some flow to the traffic at the times I use them. Whether the cameras actually work or not, they have the desired effect.