I’m assuming that you’re looking to move out for the first time?
Is £800 for rent, council tax and electricity?
The amount of money you ‘need’ is subjective based on what you want to have in your life.
You could manage a food shop for two on £30 if you wanted. It would mean you were eating very basic food - pasta, tinned tomatoes, cheaper vegetables, etc. Are you happy to eat simple food, or do you want to be able to buy a range of fresh fruit and veggies, snacks, treats, random spices no one has ever heard of Go to a supermarket online and try to budget a week of food to see what you would spend.
Do you want to be able to go out regularly - eating and/or drinking?
Do you want to be able to buy new clothes or other items when you fancy it?
What about mobiles, internet, tv or app subscriptions (eg Netflix)?
Do you need to pay for hobbies, gym or sports?
Do you have items you need for your new home? Towels, kitchen items, etc?
Will you need any insurances?
How much will transport cost you?
What about holidays or trips away?
Do you plan on saving or paying into a pension?
Make a list of everything you think you need in a month and then add it up.