Factor
I haven't read through all the stuff that others have provided, but a couple of thoughts and some answers to questions you asked.
You get metered on the water, but if you are connected to the sewage system you get stung again as they assess that a large proportion of the metered water goes back down the sewer - 90% for the utility that applies to me. It is possible to argue a different figure if you have a good reason - for example, folks with swimming pools (though they will ask where the drained water goes to) and others using lots for gardening etc.
My benchmark has been £1 per cubic metre for water supply and £2 to get rid of the stuff - perhaps United are slightly different. On top the fixed charges seem to work out at around £50 per year - same as for unmetered.
One thing you might not be aware of is the Assessed Charge system. This came in at OFWAT's bidding in 2007 and it applies if your utility is unable to fit a meter. If you live in an old house with old pipes at the supply point, you are likely to have steel (not plastic) pipes and apparently they can't or won't fit a meter at the boundary. The Assessed Charge for you would be £480 per year. I got that information from this document.
http://www.unitedutil.../Assessed_charges.pdf
Rather better than £800.
As I think others have said, you can reverse out the metering method if you don't like the outcome (though you force any subsequent buyers to have - but, hey, what the heck, you've sold the house by then). And it might just happen they UU can't fit a meter in which case you'd be paying £480 AND Mrs F can use as much water as she likes.
I only know this stuff because I have baulked this year at the unmetered charge from Thames that has gone over £500 for the same size house as you. However I built this house and the silly s@ds told me at the outset that they couldn't fit a meter - because I used the old supply pipes. So I investigated and discovered Assessed Charges. For me its a win-win, my assessed charge is £320. I'm currently trying to get it backdated to 2007.