Are those power cables running beneath the fence?

If so somewhere south of Bushey is my best guess!

No, not power cables (not in 1921!). Looks more like the girders of a skewed underbridge with the balustrade railings mounted on top, with a fairly wide (but perhaps shallow) defile containing a roadway, stream or similar running below the track. The photographer seems to be standing at the North end of the down-side girder - the ground seems to slope down just in front of the camera. I don't see how it can be South of Bushey - where are the DC electric lines?

In any case, there appear to be only three running lines, with a goods or slow line beyond the engine. That suggests somewhere near Shilton - the only part of the main line, as fra as I know, with three running lines, the additional one being on the up side.