If you're making budgets, you need a good idea about how much things cost in the first place. Food and toiletries should cost about £30.00/week, but it can hit £50.00/week if you don't shop around and it could be as low as £15.00/week depending on your diet and where you source things.
Think about the costs of travel, utilities, eating out, socialising, holidays, any kind of equipment you'd need for University (eg study materials, printers, laptops/computers, etc), and make it realistic. A well prepared student has a few thousand kept in savings for emergencies, but a few hundred is reasonable. If the numbers don't work, you might have to adjust your lifestyle to accommodate - or seek some sort of part time work on the side.
If you're an excellent student, tutoring is an easy way to earn a little extra. There are loads of student who use DSA to pay for someone to take notes for them, so that's another role you could take.