any aircraft that does not have a flight plan i am sure will get intercepted well before it enters the restricted zone
That's the point. There are lots of private owners and operators either already in the restricted zone or on the London zone boundary. The talk of concessions for those operators on or just outside the zone (Denham, White Waltham, Fairoaks, Biggin Hill, etc.), makes it even easier for a terrorist to do his deed. One could easily lift and be in central London before anyone had a chance to say 'scramble', so trying to prevent unauthorised access with a flight plan is futile. And very stupid.