Joys of the deep south, we don't get that much rain and we are on a metre.

Ha ha, you haven't tried dealing with SWW. I wanted total cut off and indipendance over a row about the high cost of water from them, snotty cow on the other end of the phone told me that i couldn't, i'd always owe them something, i'd get a bill for the water that runs off the property and finds it's way into soakaways and sewer. Oh and the reason it's more expencive than everywhere else is there is more pipes apparently.
What she probably meant was because of the lack of maintenence and investment over the last 20 years there is a huge amount of leaking pipes we have to pay for.

Sounds ambitious, going "off-grid".
Water companies typically charge you for the water they supply and also collect the sewerage/drainage charges for whoever is responsible.
We have mains sewers but get a reduction (not huge) in the latter for having soakaways for rainfall etc.

I guess if you don't take a water supply you can avoid that charge, but some sort of drainage is probably unavoidable unless you're in a very odd situation, such as your own island where it all drains into the sea.