I've been "done" twice in the last decade, one was 10 years ago in Warrington doing 34mph in a 30mph zone, and one was in Maidenhead about 3 years ago doing 31mph in a 30mph zone - both were were manned, mobile traps sited close to speed-transitions from 40mph to 30mph, and both were part of "make-an-example-of-people" type high-profile campaigns - so the tight margins are probably not typical. Normally I allow +3-4mph over limits to account for speedos showing higher-than-actual speeds plus the margins most fixed cameras have.