It's going to be more expensive in London - that's the bottom line.
It can be very difficult to get accurate figures for living costs in any given city, for several reasons, including
a) your lifestyle choices will determine your expenditure e.g. what sort of accommodation do you live in?
b) your budgeting and money saving skills will determine your expenditure e.g. do you do your food shopping at Aldi or Waitrose?
c) figures given for living costs, even by the universities, can be wrong. For instance, my university quoted the figures that the students' union claimed were the cost of living in that city. However, the SU was engaged in a campaign to increase financial support for students, and claiming that living costs were higher than they really were helped their cause. From my own experience, and that of my friends, they were estimating living costs that were about £2k higher than they really were. On the flip side, other universities may underestimate the costs of living because they don't want to put off prospective students.
It's realistic to take on a part time job and still do well academically, but be aware that
a) most universities suggest that you work no more than 16 hours a week during term time, and even this will be unrealistically high during exam periods and third year.
b) you should not expect to be paid any more than minimum wage as a student (usually £6.70ph, but this can vary by age)
c) you will not be able to earn enough to cover all of your living costs from part time / holiday work
Try and find some relatively flexible work - working for an agency (e.g. event hospitality work) and as a student ambassador are two examples of flexible work.