It’s the second of the London Marathon phone boxes!
(Coordinates 51°48’57.9″N, 0°06’28.2″E.)
Apologies that this one is very blurry – there was a water station in the way and I was trying to zoom in while still semi-running! You can see it much more clearly in the Google Street View image linked above.
It’s a bulbous-topped K2, like many of the phone boxes you see on London streets!