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Thanks for your question
You are quite right that you are paying way too much - as you are also getting charged for the employers NI and employment agengies are abusing workers with these umbreall a company set ups.
You cannot be self employed if all your work comes via the agency - you should in fact be a direct employee of theirs and they are in breach of employment legislation and you (through accepting this work through the umbreall company of IR35 legisaltion)
My advise - see if the company for whom you contract for would take you on directly (would be chaper for them) in which case get yourself registered as self employed and registered for CIS (if you are not already) and work direcly for the contractors yourself.
But I would at least go back to the agency and advsie them that you are NOT happy working through the Umbrealla company and will ask HMRC to investigate this, if the agency are not prepared to engage you as they should be - as an employee of theirs.
Let me know if you wish me to expand on any element of the answer I have provided, or let me know if you have any follow up questions