I think the soap line probably refers to the very hard water you find in the South East of England (which is basically built on chalk deposits). Unless you fit a water softener, your soap won't lather and you'll get a layer of scum on your bathwater. It's one of the things you notice if you travel down there from the North of England, and knowing Billy it's probably a metaphor for a wider social chasm.
I think the soap line probably refers to the very hard water you find in the South East of England (which is basically built on chalk deposits). Unless you fit a water softener, your soap won't lather and you'll get a layer of scum on your bathwater. It's one of the things you notice if you travel down there from the North of England, and knowing Billy it's probably a metaphor for a wider social chasm.