If you live in London, get yourself down to New Covent Garden market. Its this huge warehouse, but there are some many little warehouses there that ur bound to find someone hiring. Its huge though, u might get lost there. Its mostly work unloading trucks.
I tried getting a job there when I was 18. I almost did but the boss said I was too skinny, he thought I wouldn't be strong enough to keep up with their pace of workload. You can get paid quite a lot working there but its tough work. A lot of the people working there are ex cons and shyt so if u have a criminal record, it shouldn't be too much of a problem.