This is all quite deliberate. NATS was privatised just after the Hatfield rail crash where penalties for service standards had produced an adverse effect on safety. The Government of the day wanted to avoid any financial imperative that would have any effect on day to day operational decisions affecting safety. Hence yesterday the flow regulation could be imposed purely with regard to safety.
This takes me back…..I had a peripheral involvement with the original NATS licence and I can recall lots of debate about closure of airspace - principally that the civil service/government people never wanted UK airspace to be closed (unless they said so, of course). Lots of people involved in day to day running things said that there were very rare, but valid reasons that this might happen. But applying a 0 aircraft per hour flow rate was fine. Is this what they call spin?