That's impossible to answer, because it depends on so many factors.
My council tax is £75 per month with 25% discount, but I'm in Nottinghamshire so council tax may be more expensive there.
My Virgin Media is around £30 per month, as is my dad's, so £20 might be unrealistic - but it depends what package you have.
My water bill is £10ish per month, but I don't have a bath or garden. Yours would be more if you have either of these.
Are you going to have a mobile phone as well?
Food will vary, depending on where you shop and what you eat. I love a mix of B and M, and Waitrose.
What travel options are you having? You'll need to budget for a car, motorbike or bus fares.
Are you having any pets? Living alone can be lonely.
What about gas and electricity?
Your budget is really very basic and just based on guesswork. You need to find out what things will actually cost.