I find it differs on which gate you use at LGW I can walk through Queens gate and 9 times out of 10 the change in my pocket won't set off the detector. Tower gate on the other hand, even the smallest amount of change sets off the detector.
My most interesting one to date was a small sachet (well under 100ml) of cough syrup which I had in my pocket. Didn't think that it has aluminium foil on the inside and they went ape sh*t at me! Full pockets out, check my credit cards and passport for razor blades (even though I carry a leatherman with two lockable blades on it......) Its such a shame the staff canteen at LGW isn't airside.... long live Toni's Burger Wagon!