Red ring speeds are 20-50mph (with a red ring round them) when they're in a national speed limt then they have a 'duty of care' for what speed they do. Its all justifiable. All the emergency services abide by these rules, not just the cops.
Even if your in the speed camera photos, they measure your speed by the Markings in the road. I was told that The photos are 0.7 of a second apart, they compare both photos, and can work out your speed from the marks on the road. Your safe bud, you were only going slow!
Oh btw, the cameras are normally set to be triggered above 33mph true speed, not clock speed. There's a whole legal compliance that has to be adhered to with regards to prosecution.
