Look to the skies today (19th April 2016) as a large helicopter formation is expected over London this afternoon.
A short-notice air traffic alert to other aircraft warns of a “large formation of mixed type helicopters” flying between Iver and London Bridge sometime between 2:40pm and 3:30pm.
As it happens there is currently a wide-scale air traffic warning over the entire of London at the moment.
That is probably due to a certain US President visiting the UK sometime this week. Such as probably arriving in Heathrow at lunchtime and taking a helicopter ride into central London.
So if you see the helicopters, one of them might be designated Marine One, the name given to a helicopter carrying the US President.
Probably wont fly through Tower Bridge though.