Really? The Avery Hill campus of the University Of Greenwich consists of two sites, Mansion Site and Southwood Site, both of which are located in the 86-acre Avery Hill Park in Eltham, south-east London. The postcode is an inner-London one, SE9, and it is located in the London Borough Of Greenwich. That's London enough for anyone, surely? I know it's not central, but it's still "London".
Note the smiley faces, it was tongue in cheek
But no, I don't consider it to be "In London" in the context of taking trains "into London", in that context I'd consider the OP to mean to one of the major London terminals, as you yourself did in your first post, upon the clarification that the OP wanted to travel to Avery Hill, much of the advice given is redundant as Avery Hill is not in central London, nor located near a major London terminus.
Also SE9 is covered by the Local Authorities of Greenwich, Lewisham, Bexley and Bromley. Two of which are outer boroughs of London, so I wouldn't classify SE9 as a wholly inner London postcode either.