Did your grandmother hold any other nationality at the time of your mother's birth?
I am thinking you may be able to obtain citizenship by descent, however, I will need to check and revert back to you in the morning. I hope this is okay?
Unless you would like to discuss on the phone
I will revert back to you in the morning.
Do you hold any other nationality?
Sorry for the delayed response. I was not online for very long yesterday.
Every person who is a British citizen is so either "by descent" or "otherwise than by descent.
From the information you have provided above, I am of the opinion that you are unable to apply for British Citizenship by descent by virtue of your mothers citizenship, that she obtained by descent.
Your mother is unable to pass her nationality to you.
I hope this answers your question. If so, kindly rate the answer and provide feedback.
If you wish to discuss, please feel free to ask further questions.