toomuchicecream
Sorry that's not it at all.
OP
You become an employee of the umbrella company, they handle tax/NI just like any employer and they also handle expenses you are entitled to as a temporary employee.
I have used two. Giant are OK but you have to supply receipts for everything before payday or they won't pay expenses, they will possibly pay them the next month unless you are not earning enough, then they just don't pay.
JSA pay expenses with your pay for mileage and you only request receipts if every so often.
You have to keep all receipts because you can be audited.
They charge you for doing this but you come out with more in your pocket because you are making claims for petrol etc all the time.
Holiday and sick pay are interesting. Giant pay a bit extra as holiday pay, JSA you accrue and can ask for it to be paid whenever you want.
Giant - you have to submit everything for the 1st of the month and you are paid on the 10th of the next month - quite a long time to wait after summer for payday.
JSA will pay weekly if the agency are OK with that, they pay the day they receive funds from the agency. I did a 1 day cover and was paid the next day.
There are other companies but these are the only 2 I have worked with/for.
JSA also have other 'schemes' for self employed people where they handle the office work and one for small companies where they handle payroll.
There are other companies and some agencies will use the one you prefer.