As for what some of those outer areas of the GLA look like, here are a few which you can find on the first map in the above post.
Croydon:
Kingston (the big green space to the north is Richmond Park, and to the southwest is Bushy Park around Hampton Court Palace):
Heck, even Romford isn't entirely suburban:
The housing in these areas are generally Victorian rowhouses or semi-detached townhouses, so quite a bit denser than more of Staten Island or outer Queens. Only in the nicer parts of North London (like Barnet) do you get big family homes. That doesn't make it any less a part of the city than, say, Riverdale.
Here's the best photo I could find of St. George, Staten Island. It looks much like Kingston, although the surrounding residential neighborhoods are less dense:
