The ironic thing about knee jerk reactions after 911 was that all the aeroplanes involved were in fact on IFR flight plans and talking to ATC, so there is no reason to clamp down on VFR at all.
You could put a reasonable amount of explosive in a light aeroplane but whether you could do much damage with it is another matter. You'd do better to park up at the end of the Heathrow runways and get a stinger AA missile out of the boot and take out a fully loaded 747, or blowing up your truck bomb in the City of London....Oh yea, the IRA already did that one...........