How much do recruitment agencies charge you in UK?

Hi guys,

Can you please help with these two concerns I am having.

1. When a job is advertised for a certain amount per hour, is this what you get paid after the agency's deduction or before?

I am asking because if a job is paid 20 p/h and the agency has already took their share then that's ok. But if not, then you actually are looking at something like 16 p/h, which in the long terms becomes quite a big difference in income. The margin becomes even greater if you are paid 30 or even 35 p/h.

2. on average how much do they charge you?

Thanks in advance!