You'll have to check your contract or ask the agency in question:
You have a legal entitlement to 20 days off a year, based on working full time hours.
Some agencies include this statutory holiday pay in with the wages you get from week to week, based on number of hours worked (it'll be 8.33% of hourly rate). Some companies seperate holiday pay from regular pay on the payslip, some don't.
It's a less administratively time consuming way than accruing it and having to apply for it. However, some places still work like this.