London IS a kind of expensive place. It's certainly more expensive than some areas. I've lived here all my life so I know. I'm about to move out for uni IN London this year so I doubly know!
But if you work a part time job with enough hours and are sensible on choosing your accommodation, food and travel you should be fine.
Like if you can walk, cycle or take the bus somewhere then don't spend the extra money on the tube.
Like instead of living in those halls a 5 minute walk from uni for £199 per week... rent a room in a nice house share for £90pwk or the halls 10-15 minutes away for £140-160 pwk instead.
Spend money wisely. Don't take the tube if you can put a backpack on and ride a cycle there. Take the bus even! - It works out cheaper.
Don't be a snob with Waitrose brand stuff- Sainsbury's Basics is nearly always good enough and if it isn't you can add a bit of fixing to it and seasoning to change the taste.
Don't spend money on crap Hovis farmhouse and again buy Sainsburys Basics bread. Same stuff!