£400 a month with us would no where near cover 1/4 the normal bills and food. That's before you even get to a contribution to the mortgage or wear and tear or maintenance costs.
You could not get a room in a shared house around here for £400 pcm EXCLUDING all bills.
You're not a tenant, that would cost more. You live by your parents rules or you move out, it's quite simple. It's their house, their rules.
You're saving a lot living at home, and you should appreciate your parents are willing to do this for you. Do you help your mum with her work, which may be unpaid but nevertheless exists unless she also has a nanny, housekeeper and chef. Like childcare for your younger siblings, all the housework, cleaning, planning meals, buying food, cooking meals, clearing up? Washing floors, toilets, bathrooms?
Your mum DOES work, it's just unpaid and I'd place a hefty bet on being mainly for the benefit of you and your siblings. If they've had to cut back as a result, she's not swanning off to nail salons all day, is she?
And you have no idea of the calculations your parents have made which resulted in her staying home. It may end up costing so much in childcare, travel costs etc (second car?) for her to work that it's not worth it at this time.
It might be quite useful for you for once you do head out into the world to understand the full range of unpaid work that needs to be done to run a household and raise children and the calculations your parents made to get to the decision that your mum would only work in the home and not outside it at this point in time.