Regarding holiday and sick pay - I've had 2 situations:

One - you don't get any holiday or sick pay. Unless you are sick long term and then some kind of insurance kicks in.....

Two - your agency will deduct holiday pay from your hourly (or daily) rate and you can claim it back when you take holiday. Sick pay works the same as above.

But essentially either way, its coming out of your pocket if you take time off.