Air traffic control centres are responsible for the safety of all aircraft within controlled airspace. Not all airspace is controlled and not all zones are equal either.
Civilian aircraft travel along designated airway corridors with pre determined flight plans and schedules. Military aircraft may also use these routes but in addition have specially designated areas for low flying and weapons training. They can of course also fly in uncontrolled airspace.
Most general aviation flying takes place outwith controlled airspace as much as possible except of course while taking off and landing at an airport. Airports themselves generally have a control zone although the shape size and heights involved can vary greatly.
All factors are considered and classifications of airspace are set be the Civil Aviation Authority.
There may also be other restricted or traffic free areas listed with or without conditions. Firing ranges and nuclear power stations may have permanent no fly restrictions for any aircraft. On the other hand an area around a prison may only be off limits to helicopters.
Restrictions may only apply for one day for an air show, royal flight or country fair etc. These temporary restrictions are sent out as official notices to airmen or NOTAMS. Every pilot is responsible for planning the flight which includes referring to notams for his route. It is also a legal requirement to carry an up to date air navigation chart on every flight.
Most pilots will be in radio contact with a controller on the ground but that is not always the case. Likewise not every aircraft carries a transponder to identify it to radar control or other aircraft.
During the course of a single flight a pilot may be in contact with a ground controller, tower, approach and a radar advisory service. The idea of each and every one is to allow aircraft to move safely to their destination.
Not all controllers use radar screens although every inch of airspace is monitored. Control tower staff use a system of slides with aircraft details and callsigns then look out of the window to see where they actually are on the ground or on the approach to land.
Approach controllers speak to aircraft from about five to fifteen miles out to give them a slot to move safely in to or away from the airport. They would then hand over to the next terminal area, a radar advisory service or the local tower.
Upper airspace and the approaches to UK airspace are monitored by both civilian and military staff to provide assistance and information as well as to identify possible threats.
Unidentified aircraft could be anything from a probing mission by a foreign military power to a drug smuggler or an aircraft in distress.
Emergencies are dealt with by a specialist team called a distress and diversion cell. They maintain an overview and arrange for priority routing and clearance for aircraft in trouble. They can also assist pilots who become lost or disorientated by fixing thier position accurately on radar.Air traffic to home.