I am not saying you are not entitled to maternity leave, I am saying that is the view the UKBA are likely to take.
If your employer has informed the UKBA that you are no longer under a contract of employment, the likelihood is that the UKBA will cancel your existing visa.
If you are in dispute with your employer, the first thing you need to do is raise a grievance through your employers human resources department. The next thing you need to do is inform the UKBA of the situation between you and your employer.
If they still require your passport, I am afraid there is little you will be able to do.
The alternative is that you seek alternative employment with an employer that has a UKBA Sponsor License.
Unfortunately, the dispute you have with your employer is of no concern to the UKBA who generally apply the rules fairly strictly.
The rules state that you must inform any change in circumstance to the UKBA. As a result, and from the information you have provided above, it seems your employer, for some reason or another, has decided to terminate your employment as opposed to grant you maternity leave. This is obviously wrong.
I would suggest you first contact ACAS. You will be able to find their details on google. Once you have done this, you should instruct a Immigration Lawyer to contact the UKBA on your behalf.
If you wish to discuss, please feel free to ask further questions.