It's not just ourselves we have to protect, it's you guys too! We have a duty of care over you all. If we have several of you in the zone and, heaven forbid, two of you collide, it's highly likely that you can all kiss goodbye to flying over the city full stop. We have to be able to pass traffic information to you all, in order to assist you to see and avoid. Sometimes, we might have 5 (or more) of you in close proximity. That's 20 pieces of traffic information that need to be passed if we are doing it properly. That takes time, and unfortunately, we aren't only there just to do that.
Surely that is the sole reason for the existence of ATC!
Point is though, in cases where transit access is totally denied, as was here, pilots must route around and will most likely go by the shortest distance around the outside, in Class G. This results in the situation where aircraft will be following the same route in opposite directions and at very similar altitudes with no ATC input.
There are already a number of choke points in the London area, especially at that end of the CTR. Closing swathes of Class D creates less safety, not more.