Try the Erith (Kent) and Slade Green area if you really want a dump. When I was a kid, Erith had quite a nice little town centre with plenty of shops (including a department store called Hedleys) along Pier Road, but they flattened it all and quite frankly except for the walk along the river it's not worth going there these days.

Incidently the locals pronounce it's name EARITH not EHRITH as they always seem to on the BBC.

I have often heard Gravesend called Deadend.