The “Y” arrow sign exists in only two places which I know of; M25 Junction 5 where the sign was an experimental design due to the fact all M25 traffic has to exit the motorway to remain on the ring road (https://goo.gl/maps/mZqAp273rfm) and the aforementioned M6 example.
Arguably, the M25 example is slightly misleading as the exit layout is a tiger-tail diverge without the chevron area (instead a Dia 1004.1 is used) (https://goo.gl/maps/sgHfH2i3PS22).
The M6 example (https://goo.gl/maps/Auo3f4Dthnx) is more accurate as there is no tiger-tail type diverge; the centre lane splits exactly as the arrow shows. I’m curious as to if this layout performs better than the conventional tiger-tail as you regularly see vehicles cut across the solid chevrons because they think they can’t exit from Lane 2; a notorious example of this is on the M56 approaching Manchester. The “Y” lane split type diverge layout is extremely common in Europe and the USA.
The forthcoming revision of Traffic Signs Manual Chapter 7 will have examples of the use of these Y-arrows, but no dimensioned drawings.
At Buchanan Computing, we have implemented them in our SignPlot software by scaling them off the drawing in TSRGD. We make the arrows 36 sw high, and the 2-headed lane bifurcation arrow 24.5 sw wide. We follow as closely as possible the same rules for vertical and horizontal spacing as for other dedicated lane directional signs.
Since 2016 TSRGD came out I’ve been looking out for these signs on the road network, but have only so far noted one (on the northbound split between the M6 and M42(S)).