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.