Sven, you have a well argued point, but what can be done?
One needs to assume that the terrorist is going on a suicide mission. (Hard to do any damage with a light plane without killing oneself).
Should we assume he will first learn to fly?
If so, that means vetting at the flying school - like they do in America (TSA). That's not a big deal, but obviously it will detect only individuals who are either a) known to the security services or b) have a dark skin, curly hair, a beard, and walk wrapped in a bedsheet with "Death to Britain" on it while handing out leaflets supporting Al Queda and demanding that the pretty reception desk girl should cover up her t*ts.
The American vetting system (which I have been through, well and truly, having poked my fingers up the dark orifice of every fingerprint machine at the U.S. embassy on my FAA PPL/IR/CPL stuff) relies almost totally on the assumption that no born and bred American citizen will want to hit America - something which is a fair assumption in America (99%+ of Americans are genuinely proud of their country) but certainly not in Europe where - post 1939-1945 - patriotism is equated with nationalism which is a dirty word.
I have never met anybody who thinks the TSA measures actually do anything other than look like something is being done. Possibly they keep out the really obvious Muslim extremists whose money would previously have been welcome at any U.S. flying school? The TSA is completely ineffective against any American person unless he already has a record.
If OTOH we are to assume that the terrorist is not bothered about getting a PPL (easy to get airborne in a light plane if you have sim practice and some ground training) we need to look at aircraft security.
Securing airfields is impossible. Even Gatwick/Heathrow would be trivial to penetrate, in the dark, with wire cutters. It is only a fence after all. It would need to be double, electrified and have a mined strip between the fences. In any case, a vast number of people at LHR etc have airside access; many doing low grade jobs which tend to be done by foreign workers so going on "obvious appearance" gets you nowhere.
One could put locks on the planes e.g. padlocked chains over the prop. That is probably practical and is already done in the USA, I gather.