I guess that pre-grouping LNWR and GNR didn't compete that much, whereas both hated the Midland - so anything they could do together to frustrate the Midland was worth thinking about.
The GN and LNWs hatred of the Midland, didnt stop either of them from running joint lines in partnership with the detested enemy: but that was true of a fair number of mutually hostile railway companies in pre-1923 Britain. One of the many endearing weirdnesses of that scene...
A GN / LNW partnership feature which has always appealed to me, is the system part GN / LNW joint, part purely GN serving the countryside of east Leicestershire, and Nottinghamshire: from the map, looks highly enticing and one suspects, instituted at least in part to put a spoke in the wheel of the Midland, which was also prominent in that part of the world. To my regret, my first-hand experience of that system at the very end of its life was minimal, and did not include any travel. Oh to have been born a few years earlier...
"Railways of Hertfordshire" gives a bit more information on the construction of the Hertford Loop. Wood Green to Hertford was authorised in 1865 and opened as far as Enfield by 1871. Enfield to Langley Junction was authorised in 1898 and land purchased in 1901. The entry then reads:
So -- per quoted item -- the Ian Allan "PGA&G" essentially has it right with the Hertford Loop -- running, freight-only, through to Langley Junction by 1918: I see that said atlas shows Hertford -- Langley Junction just as "plain route", without the two passenger stations which it acquired when opened for passenger traffic.
I find that in the main, the "PGA&G" is extremely accurate as regards its stated mission. Am a bit dubious about its showing, as at proclaimedly the end of 1922, the North Wales Narrow Gauge Railways (soon to be the Welsh Highland), running south of Rhydd-Ddu (South Snowdon) to terminate at Beddgelert -- but gen which I can find, gives no certain answer on this; maybe the Atlas has it right.