please help - termination nightmare!!

[NotAllThere]

You can have more than one employer. In a normal work situation, your contract with an employer would stipulate you need their permission to work for someone else. The way you can check this is to read your contract. What does your contract with the umbrella company say? If it says nothing about needing their permission to work for another company - which is extremely likely - then you haven't got a problem.

In the extremely unlikely event that they give you any grief, post here and you'll get more advice on how to handle it.

As far as dream jobs go... do you remember some of your dreams...

[LisaContractorUmbrella]

An umbrella company is the same as any other employer - you work under a Contract of Employment; if you want to terminate that contract, you need to give notice. It would be highly unlikely that the contract contained any sort of covenant preventing you from working elsewhere but you should check.
Just a note for the future - contracts are legally binding and you should never enter into one without first having read it.