The good news is that this is a training exercise. An RAF Sea King helicopter, together with a Police search helicopter, will be in the Mapledurham/Purley area of the Thames river working with emergency services.
I am told that the helicopters are expected in the area at about 11:15 - 11:30 with the exercise commencing at 13:00.
I trust everybody will support this training mission and short-term inconvenience as it is for the greater good of ensuring preparedness for future real-life emergencies.
Update: You can read coverage of the event in GetReading
Update: You can read coverage of the event in GetReading