I've updated the road classifications using OS Open USRN (August 2023) and the OS OpenMap Local (October 2022) layer.

Numbered Street USRN 22700030 appears to correspond to the B121. This covers the following sections:
Stepney Green
Stepney High Street
Belgrave Street (Stepney Way to Lighterman Mews)
White Horse Road (Matlock Street to Commercial Road)

There is a small section which does not have a Numbered Street USRN, but is shown as a B road in OpenMap local:
Belgrave Street (Lighterman Mews to Troon Street)
Troon Street

All of the above are now mapped as highway=secondary.

The section of White Horse Road between Belgrave Street and Matlock Street has no Numbered Street USRN and is now highway=residential.

The section of White Horse Road between Matlock Street and Troon Street having Numbered Street USRN 22700030 makes no sense whatsoever, but I suppose it's still technically part of the B121 until someone from LB Tower Hamlets updates it in the National Street Gazetteer.