An umbrella company offers a simple service.
A business is going to pay an individual to do some work for it. The business judges that the individual’s role amounts to that of an employee, who should properly be paid via PAYE. The business doesn’t want the admin of doing that themselves. The umbrella company offers to employ the individual instead, and invoice the employer for the salary and the other associated employment costs (NI, pension).
Or, a business is going to pay an individual to do some work for it. The business, considering it to be an employment role but being unwilling to put the person on PAYE, tells the individual that they must invoice them via some other entity, most commonly a limited company set up by the individual themselves for that purpose. The individual can’t be bothered doing that, or knows that in the hands of her own company the income will fall under IR35 and be subject to full employment taxes anyway. Instead she decides to sign up with an umbrella company, who will receive the fees, operate PAYE on them, and pay her the net.
What’s in it for the umbrella company? A fee (most commonly suffered by the employee, even if the business insisted upon the arrangement, naturally!).
They’ve had a chequered history. You can see that in theory they’re simply a vessel to get fees from A to B, with the appropriate employment taxes paid. So they should all end up with more or less the same split of money between the worker and HMRC, with their commission being the only variable.
However, market forces meant that they began to compete with one another, to try and offer a bigger share of the split to the worker and less to HMRC. This was never achieved by legitimate means. A favourite trick was based on a wilful misunderstanding of the rules on something called dispensations. A dispensation was an arrangement with HMRC, where HMRC said - for example - “If you reimburse an employee for legitimately incurred and legitimately tax deductible travel costs of up to £30, don’t bother telling us about it, we don’t want to record it as both income and deduction for the individual, it’s a waste of time as it ends up at zero”. Then, regardless of whether any such costs were actually incurred or not (they weren’t) the umbrella company would include a tax deduction of £150 on every week’s payslip, with a consequential suppression of the tax deduction truly due.
Eventually, HMRC got wise to this. Those schemes were exposed and shut down. Any time you read anything promising you materially greater take-home pay than similar providers, it should always have been clear that this could not be achieved legitimately, but now - crossed fingers - we’re back to an acceptance that umbrella companies are machines that process the same inputs in the same way and should, consequently, churn out the same outputs.
A rather long-winded way of saying - it shouldn’t matter, they should all be pretty much the same!