Most students from outside the UK will be given a Student visa, which should allow for part-time work of up to 20 hours per week during term time. There is no restriction on hours outside of term time. Allowances and restrictions to work will be outlined on your Residence Permit. Please be aware that student visa holders cannot be self-employed or engage in business activity, see https://www.ukcisa.org.uk/Information--Advice/Working/Student-work#layer-6647h
For a discussion of these issues, read: https://www.ukcisa.org.uk/blog/6257/A-working-definition and if in any doubt about the immigration implications of possible employment, please check with the International Student Advice Team at your university.
You can find some tips on finding work at: https://www.ukcisa.org.uk/Information--Advice/Working/Finding-work-employers-and-tax#layer-3765
See also International Student House's blog on Finding a Summer Job in London