"Clearance for Special VFR flight in the UK is an authorization by ATC for a pilot to fly within a Control Zone although he is unable to comply with IFR. In exceptional circumstances, requests for Special VFR flight may be granted for aircraft with an all-upweight exceeding 5700 kg and capable of flight under IFR."
Would an 'ATC computer failure' really be considered an 'exceptional circumstance' sufficient for CAT to depart the Class A CTR at 'LON' (or do you mean LHR?) en-route to Brussels? Or was it a small aeroplane?