The devices that detect radar are illegal in the this country although are still available on a few websites (foreign companies i believe like Snooper). Those devices are pointless as it only tells you off the radar once its already hit your car / pointed towards the car, by then its far to late.
I wondered when someone would point this obvious point out.
If you got stopped by the police (for something like a broken rear bulb or a random spot check) and they checked your record and found a radar detector in your car, you'd be in for trouble.
It's not worth it.
The majority of fixed speed cameras are on most sat navs, so if you're on unfamiliar roads, just turn it on and you'll get a warning (mine tells me when there's one upto 1km away).
If it's a cop with a radar gun or a "talivan" then you'll just have to leave it to a) your ability to spot it and b) luck.
I know what you mean about going from a 40 zone to a 30 zone.
Myself and my mum both got caught while driving in convoy because an unmarked police van was sitted over the brow of a hill where it went from 40mph to 30mph. I was caught at 34mph.
Well done on passing your test, not every experienced driver would be able to pass the test again without doing lessons.
Now just be sensible and enjoy driving.