Olympic Airspace Restrictions!
Yes, from 13 July 2012 to 12 September 2012, you won't be allowed to fly around a huge area extending out from London as far as Duxford and Old Warden without complying with a host of restrictions. One of these restrictions is that you must have a transponder and squawk an appropriate code - many of the aircraft that display at Duxford and Old Warden are not transponder equipped, and it is not economically viable to fit them with one. Now I suspect in practice based aircraft at these fields may be able to take part in displays if an exemption is negotiated, but how will such aircraft transit from those fields to other venues, or vice versa? They won't be allowed to.
So two months of crippling GA flying so that two weeks of running and jumping - on the ground - can be 'safe'. I bet any terrorist who was planning on loading up his Cessna with a few kilos of fertiliser is already sighing and replanning a different style of attack now that there are going to be airspace restrictions that make such an attack officially against the rules.