Unfortunately this is a classic case of the agency not being clear with you. For example, you may have agreed with the agency that your rate will be £140/day. Any reasonable person would expect that this would be your gross pay -- like you'd see as a salary on a permanent job ad.
But because the agency runs payroll through an umbrella company, they palm off the breakdown of pay to the umbrella. The umbrella then says that in fact the £140 includes an 'uplift' from what the day rate would actually have been if you were paid PAYE by the agency. The 'uplift' is generally the statutory 13.8% Employer's NI contribution that the umbrella as your employer has to pay to HMRC. This means that your actual gross rate of pay is not £140/day, but actually 140/1.138 (the 1.138 is the 13.8%), so your gross rate of pay is, in reality, £123.02. You ought to consider whether, if it were clear from the start, you would have agreed to work at that gross pay rate.
In our experience, when pushed on this point, some agencies simply say that you have a separate agreement with the umbrella, that there's nothing they can do, and so wash their hands of the matter.
We consider this to be a misrepresentation of your gross earnings by the agency and we are campaigning to highlight this practice -- it's very widespread, sadly. Essentially, you may have thought that you'd agreed £140/day, but actually that rate includes the employer's taxes -- so you are paying your umbrella company's tax bill -- and you actually agreed to £123.02/day, a sum that is then subject to your tax bill.
When we highlight this many teachers are understandably furious -- so it's definitely worth asking your agency for their Key Information Document and examining it carefully to ascertain how pay is broken down. Agencies often prefer teachers to be paid via Umbrella companies as this reduces their administration costs and limits their liability. Teacher Booker does not use external Umbrella Companies, instead, we pay simply and straightforwardly via PAYE -- read more here. For comparison, Teacher Booker's Key Information Document is available at our website here.