Those average speed cameras are usually only in construction zones on the motorway where the lanes have been narrowed and speed reduced to 50mph, and they have a fairly generous leeway as well. I've never been ticketed for driving 54mph (actual speed, speedo shows 59mph) on cruise through these areas for years, and you're constantly overtaken by people going at least 3-4mph faster... who knows if they get ticketed though. There are also fixed speed cameras, red light cameras and CCTV cameras all over the place for your surveillance pleasure.
Don't pick on the UK too much though, if you drive in France on the Péage (toll) motorways, I've heard they clock your time in and time out, and at the exit tolls your speeding fine is payable on the spot. Most of Europe has on the spot fines, but it's usually tens of Euros, not hundreds of dollars like in the US or Canada.