I know that is the current convention, but it is not a very helpful one.  Most of the people who live in the outlying boroughs of Greater London - the ones that were 'grabbed' when the GLC was set up, like Romford (Essex), Bromley (Kent), and Richmond (Surrey) - continue to use 
the old country names, and will probably do so for another hundred years.  I live in the London Borough of Croydon, which has been a London borough for much longer than Barking or Romford, but many people still refer to it in their postal address as 'Croydon, Surrey'.  I think to avoid confusion articles like this should at least make it clear that the old names are still in common use.  Otherwise people will come here after Googling 'Barking Essex' and then wonder if they have got the right Barking.109.158.132.88