Despite London being gone, there's still parts of the city outside the blank hole where it used to be.
It's no wonder they ended on this episode: With London (and supposedly parts of Earth) destroyed, what's left to do - no London, no Dangermouse.
The plotline of this episode bears a lot of resemblance to a one liner from the Hitchikers Guide to the Galaxy. The quote goes thus...
(Arthur Dent and Ford Prefect are talking about the fate of Earth)
Ford: Better then some, I was heard of a planet that got used in a game of intergalactic barbiliards. Got potted straight into a black hole, killed ten billion people.