It all depends on your circumstances - someone living at home with their parents would need less money than someone supporting a household. Also you would also need to think of housekeeping and clothing etc. You should put down what you spend.
The other thing to remember is that you are writing off debt with bankruptcy. If you could pay off all your debts with £350 (and that I am sure is the maximum you have available) in three years then bankruptcy would not be the route to follow as you could pay off your debts. If it is more than that then bankruptcy - with its six year write up on your credit file - needs exploring.
Phone one of the free advice agencies - NDL, Payplan or CCCS or contact your local CAB to go through an expenditure with you and discuss the options available. These forums are good for support and can advise but there is nothing better than talking to someone.