I was offered that from pertemps and there was something like you paid a deposit in case you didn’t show up which you got back if you did turn up. As you said above it was for their insurance. I never did it as got a permanent job first but sounded reasonable enough.
Is there anything like a Stobarts, Downtons, Turners etc near you? All are known for taking new passes. Not the best payers I admit but will give you a start and a steady income while you get some experience