As it's an advisory lane which is part of the main carriageway, it's already tagged there, e.g.
cycleway:both=lane
cycleway:both:lane=advisory

Cycle lanes aren't rendered on the default map style (OSM Carto), but they are on CycleOSM, OpenCycleMap, etc.

I would suggest reverting your edit, as there isn't a separate cycle track along most of the length of Leytonstone Road.

Documentation on cycle lane tagging is linked below.
https://wiki.openstreetmap.org/wiki/Tag:cycleway%3Dlane
https://wiki.openstreetmap.org/wiki/Key:cycleway:lane