If anyone tries to make something simple sound complicated, then you can bet you are being palmed off with something that’s not for your benefit somewhere along the way.
I signed up with 9 agencies last year, and have been booted off some of them because I won’t work anything other than PAYE.
Seems crazy really doesn’t it?
“Hey boss, I don’t want to evade tax which would be illegal, and I don’t want to pay some third party to try to evade it on my behalf either. I’d just like us all to be honest in this august business.”
Boss: “Here’s your p45”

6 out of the 9 agencies I signed up with never gave me a SINGLE assignment, despite me signing up during the “busy buildup” time of November.
I put that lack of work more down to “not being prepared to do umbrella” than not being qualified/experienced/available enough in other ways.
FFS Nights & weekends are my thing, and I still get diddly squat from these shysters!
I imagine the big signees to umbrella are the 9 points brigade, or those who can’t work out or read on this board that umbrella is all a big fiddle twice over.
You pay a commission, and they’ll give you a tax rebate you’re not entitled to to justify the expense. Then, after the end of the tax year, you have to pay it back to HMRC when they decide you were not actually eligible to get such unreceipted offsets.
If you don’t work in at least 3 seperate locations per year, you’ll be paying back all the mileage claimed as well!
Odd, when the first thing the umbrella pushers like to say to you is “We can guarantee you 40 hours per week once you sign up”.
(Driverhire, TRG for example depending upon which side of the river you are!)
Yeh, 40 hours on the shifts you don’t want, and the place you don’t want, doing the job you don’t want. In my case, I was offered monday-friday 6am starts 8 hours a day at a certain depot quite near the dartford tolls… I wasted a day going through the application process only to find out it was all this big con designed to palm people off with the less desirable work. Does anyone else commute umpteen miles for less than £10ph for a flat shift, and a pay-per-payslip plan that encourages you to work multiple short shifts thus losing you the greatest amount in overheads… Garrrghh! 
I turned up because they were keen that I was “willing to work nights and weekends” (for which they touted rates like £14ph) on that basis - so I thought.
Of course, the plummy hourly rates are not available - unless you’ve put in your 40 hours monday-friday FIRST. Essentially, it’s overtime for the boys and NOT for those who pick and mix shifts which is what agency work is supposed to be about from the driver’s point of view - not just mine! 
I’m quite happy sticking with SMS Mainstream now, as essentially they do what it says on the tin, which is all any of us should want from an agency.
Holiday pay, No fancy differing hourly rates at different times of the day, and the same rate all the way through.
PAYE as standard, with mileage and meal allowance offsets submittable each and every week.
Essentially, you get everything that an umbrella firm charges £27 per payslip for - but all for free, complete with holiday pay that is NOT built up by a deduction from your gross wages. Getting this alone is worth a lot more than an extra pound an hour that seems to be commonplace among the umbrella brigade. For someone like me, who wants to work loads of odd shifts rather than weeks and weeks at the same place, not paying per payslip is an absolute MUST.
For the equivalent of SMS in other areas beyond kent, I suggest looking for whoever supplies mainstream with drivers to the big boys.
I know they are eager to get some 17t daytime drivers on the books for the food deliveries, as I’m guessing there are just not enough younger fitter drivers around these parts to fill the demand!