For understandable reasons many people would prefer to organise a job before they come to London. Although this may be possible to do it is more realistic to come to London first and then look for a job.

Why is it better to come to London first before looking for a job?
i. Nearly all employers prefer to meet the person they are considering employing before offering him or her a job.

ii. Generally speaking in England less importance is give to diplomas and qualifications than in many other European countries. If you send a C.V. listing your academic qualifications this may not be as impressive as it might be in your country. In any case few employers will have the time to read C.V.s.

iii. Employers typically need someone to start immediately and this will only be possible if you are already based in London.

Is there any way to guarantee me a job before I come to London?
There are several organisations that promise work, usually for a fee (payment). It is worth being aware that the reality of the job is not often not worth the money paid; often guarantees of a job are no more than the guarantee to an interview with an agency.