working for the first six month off-campus will be illegal. On second chance of you being caught, they will deport you, sometimes they do so on first instance as well. So going for off-campus job, not really a good idea. However, check with your college if you can fetch on campus job for first 6 months. Check college's website for job postings (internal) and then contact the International Student Center of your college.
Secondly, I know, you must be from India or Pakistan, so am I. So we usually need a job during first six month as well to cover our expenses or to cover tuition fee. But there is no legal option. As said earlier, illegal option is very risky, but depends on situation, some people are willing to go for the risk and most don't. You should probably wait till you land in London next month and then check with friendly seniors, they might help you out, hope you understood what i meant.
Cheers!