Ken the name of the unions or rep group is a separate issue to adverts.Only ads allowed are on the small panels.
I remember some wiggle room for wraps but it's so long ago I can't find the relevant section.
This is the important part...
'Advertising, except electronic signs, is generally allowed in
or on taxis and wheelchair accessible taxis.
Any advertising must be inoffensive.
The door signage (branding) must be the only feature on
both front doors, i.e. no other logos, branding or
distractions.
Electronic displays (including messaging displays), visible
from the exterior are not permitted unless with prior written
consent.
Only the official card acceptance logo may be displayed, see
Fig 3, Page 56.
Signage/adverts displayed on rear doors cannot exceed
1600 cm2
in area (per door)'