As long as your "...at Kings Cross" includes about 0.5 - 0.75 miles northwards, I feel sure you are right Bryan.
The opening cine footage is, I am sure, at what is mostly called 'Belle Isle', the area between the first two tunnels (called Gasworks and Copenhagen), with the scene being the north end of the former and showing a Down train heading away from King's Cross on the Down Main No.1, at a time which pre-dates the 1932 complete resignalling of King's Cross using Siemens power-operated equipment worked from the 232-miniature-lever signal box built next to West box at the end of platforms 5/6 (renumbered 4/5 in the '70s), and with which many of us will have been familiar until its circa 1976/7 demolition.
The visible signals to the left being an upper-quadrant (UQ) subsidiary signal, which would have had above it, out-of-shot, one of the very rare (in the UK) full-size UQ 3-position signal arms: Both being Kings Cross West box's signals (power-worked) applying to the Up Relief line, the Up line in the centre bore of Gasworks, and which were only provided in, I think, 1923 when this line was converted from its former Up Carriage line status.
To the centre and right, the lower-quadrant arms which we see the back of, on gantry-mounted 'suspended' dolls, are Belle Isle Down box's Home signals with Distant arms beneath for Copenhagen Junction/Goods & Mineral boxes.
A short distance behind the cameraman when filming the above, stood Belle Isle Up box (BIU), with its four 'splitting' Home signals (unseen in this film clip; 2 per line) for the Up Fast and Up Slow on a gantry not many feet south of the box (and note the brief close-up scene of two fogman's signal repeating indicators bearing the line-names "UP SLOW" and "UP FAST").
From about 1.11 in the video, we appear to see a north-facing view of the 'Fog Fellow' 'fogging' for those BIU Homes (with what would fit as part of the bottom of BIU box behind him), plus, the far background, through the overbridge (which would be the North London railway viaduct ; and including lines curving correctly for this being 'Belle Isle') includes what appears to be a signal box, with an adjacent gantry of four closely-spaced signals: These features would fit for being Copenhagen box with BIU's 'splitting' Inner Distants (also 2 per line).
When we see 'Fog Fellow' pulling his lever (I think there's only the one), with two lower objects just nearer the camera which I suspect are those two fogman's repeaters, it would make sense for that to be his operating a magazine detonator-placer for the Up Slow (the further away of 'his' two tracks), with him being expected to carry on placing detonators for the near road (Up Fast) by hand - quite normal for those times.
Hope that's not too much analysis for you !