Your lease will set out what counts as a communal area (unfortunately I can’t visualise your particular circumstance, so can’t comment further). Landings outside your property to which others have access are very likely to be defined as communal.
The Service Charge guide, page 16 has a grid defining what is and isn’t Camden’s responsilibity. From my experience and from looking around my locality, I would say front doors are leaseholders responsibility (including frame if necessary), but I’m no lawyer! Weirdly, internal door frames are Camden’s responsibility.