Part of me suspects it is done this way for political reasons. Nobody wants a sewage reservoir near their house. Everyone is scared of a reservoir potentially bursting and flooding a whole district with sewage.

Whereas if you say "it's a big pipe to take sewage away", nobody can object to that. Everyone wants their sewage taken away.

The fact the pipe is also acting as a reservoir becomes a technical detail.

In the southeast of England where would this storage reservoir be?

It’s been over 50 years and we still haven’t built the only proposed reservoir in Kent at Broad Oak near Canterbury. We are useless at getting things done in the uk. That a third Blackwell Tunnel is being built is incredible.

It's raw sewage, so probably not on the surface.

You'd dig up a few acres of park and put it underneath.