On a given 150nm flight in the south of England I might get 50 traffic reports (from say Farnborough/Brize/Shawbury) of which I guess 80% are "unknown level". How useful that is is a matter of opinion but it's an awful lot of resource wasted, on both the ATC and the pilot end of things.
From those that I do spot, and from the fact that I usually fly as high as I can without getting an IFR clearance into Class A (which complicates matters unnecessarily) I "know" most of them are way below me.
By "local bimble" I meant flight within a narrow range of one's airfield. With most UK airfields, one cannot venture very far in any one direction without reaching CAS. If Mode C was mandatory, you can bet that almost every plane that has a normal electrical system would have a Mode C fitted - simply because it would otherwise be all but useless for training or for self fly hire. If one had the US-style Mode C veils, while it would still be possible to fly a long way without needing a transponder, in practice one would choose to not restrict the utility of the plane to such a degree.
It's curious to observe that in America, often described as the land of aviation freedom, they have mandatory Mode C in the "obvious" places, while in Europe everybody has been fighting it.