Mmm, in the UK they have to take the lowest speed measured in a period/distance (I think on the road it's 1/4 of a mile), which means braking sharply may well mean that you don't get a fine/points, even though they know you were going faster simply because they can't take the peak speed.

They'll still stop you and use excessive amounts of sarcasm though.

Hugo
(who once got off with 125 in a 50. Whoops... well, I had no idea it was a police car following me - it was very dark - and I'd just had the turbo fitted to the MX5 and, well, he couldn't keep up... (good job, he'd only got up to 100 trying to follow me) after that point I started driving a bit slower)