The ANO requires operators of SUA of greater than 7 kg to gain permission from the appropriate ATSU to operate inside ATZs, Class A, C,D or E airspace. (See CAP 393, Article 166(4). The general thrust of NATS (UK) policy for the large international airports (as explained to me when setting up my own business last year) is that sub-7 kg SUAs may operate without restriction in CTZs whilst above-7 kg SUAs will be refused permission on an ad-hoc basis but may apply under Non Standard Flight procedures. Obviously smaller airfields may have a more relaxed policy. At Manchester airport, the locals release flocks of pigeons on a regular basis, judging by the NOTAMs, which is arguably more of a danger to traffic than a single SUA. But NATS accommodates it and everyone gets along just fine.
With respect to operating at the end of an active runway, Article 166(2) would come into effect whereby "the person in charge of a small unmanned aircraft may only fly the aircraft if reasonably satisfied that the flight can safely be made." So if you do that then you are breaking the law. Operating at 300 ft agl 5 miles south of Gatwick where the airliners are all up at 5-7000 ft should not be a problem.
As someone wisely said above, there is plenty of airspace to share around quite safely. Spouting off about regulations that don't actually exist really doesn't advance the debate.