UK - can you be arrested for small amount off cannabis

[mashed]

swim nearly got arressted last night.
swim was in the back of her friends car and her two friends (male) were smoking a 5 skinner between them swim doesnt smoke cannabis though when the driver accidently (and very stupidly lol ) cut up a a traffic cop the passenger then put the joint out a hide it. the copper pulled us and came over (with the usual aggressivness some off them can have) and procceded to talk to us like scum.he said he could smell cannabis and if we didnt give it too him hed lock us up.he want back to his car and said we had 2 minutes to dscuss it.so the boys gave the cop the joint.he said he know the driver had more (which he didnt) and then asked did the boy who was smoking it have any convinctions for cannabis.he then gave him a street warning and the driver a £30 fine.
so swims question is they allways threaten to "lock you up"
unless you give it too them and own up
can they really do this for a small amount?
thanks