I'm guessing that the agency may be liable for NI if you are working through an umbrella company - I suspect that you signed on with them accepting their terms. It may be worth googling umbrella companies and emailing to ask the law - however, if you don't comply with the agencies terms, you may risk any future contracts with them.
I am trying to read into your post a little, so my reply may be a little off.
I am assuming you have signed to an agency who has placed you on the contract you are on, I would imagine if this is correct that what they have told you is that you are not able to sign to another agency at the same time and accept work through them (working at the same time as this contract) This would be because the agency you are currently working for are dealing with your PAYE (you wage) and are sorting all of your tax and NI for you, if you are working through another agency at the same time then it would knock out all of their calculation for your tax payment ect.
Through what I know an agency is an umbrella company introducing you to a company for contract work, so to sign to another agency (umbrella company) as said would not only cause issues with PAYE but also cause a conflict of interest which any agency (umbrella company) wouldn't want.
As actd has said you would have signed to agree to the terms of the agency's terms so you shouldn't work for another company at the same time or they would most probably terminate your contract and not offer you any work in the future.
Know this is an old topic but to shed some insight threat may help, and may be well off as I am reading into what OP said but if you have a contract through the agency they will get paid and pay you the agreed rate. Now umbrella company's are alternatives to setting up your own limited company so they take care of NI, PAYE setting aside sick pay etc for you.
When I was once approached and was about to sign a temporary contract with a company the company hr dept pulled the plug as they knew it was not realistic and insisted on the company finding a perm employee. Long story short after some negotiation they made me a great offer I took. I found out later from the hiring manager that the agency were taking 100% of the daily rate they were originally offering!
To use an umbrella company your contract I believe would have to be direct with the company and not the agency so they would lose out a lot depending on what your doing and rate.