on yer bike

when I was stopped a couple of years ago, they claimed to stop me to check the bike wasn't stolen... Could they not have set up a number plate recognition camera further up the road, and only stopped me if it came up as stolen? they then went round my bike to check the "legality" of the bike.

My question is do you legally have to stop, surely they can only legally stop you if you've been seen to be doing something wrong? I wouldn't want to be the one to test out driving around the officer pulling me over but it is a thought.