*sigh* I'm 24 and had to go live with my nan at age 15. Unfortunately a lot of the people at SFE helpline are not aware of how to deal with this kind of situation.
It frustrates me to hear people still are given incorrect or partial advice. Based on my experience, here is the best advice I can give you. If any of it doesn't apply to you, apologies. I just prefer not to assume anything about your home life.
Firstly, it is measured by your household income, not your parents. Your circumstances and reason why you came to live there will affect SFE decision. You shall need to supply proof that you have been living at this address and for how long.
If you chose to live there, then your parents income will be considered as SFE will argue you could choose to move back.
If however your family decided that you should live with your grandmother, then they should be assessing your household income.
The difficulty SFE have is they need to fit you into a neat box, either as estranged from your parents or financially independent. Your grandmothers income cannot be assessed, this is why they are trying to hone in on your mothers income even though it violates their own policies.
If you are age 25 or over, then you are classified as financially independent.
If you have been earning over £8,000 a year for the past 3 years, then you are classified as financially independent.
If you have had no contact from your parents or they have not financially supported you at all in the last 3 years, then you are estranged. It requires that your parents relinquished all parental responsibility of you and this is a permanent decision by them.
If when you moved to your grandmothers, your mother paid any form of maintenance towards your upkeep to your nan, then you just state how much was paid per month and when or if such payments ceased. This counts as household income. If this has been paid in the last three years, then you are not estranged.
I suggest you download the SFE guide to funding and applications PDF by searching for it on Google. When you ring up SFE, if they disagree with you, refer them to their own policies.
When you apply for SF online, you may need to have SFE on the phone at the same time like I did. I found I wasn't able to move onto the next page without the SFE advisors assistance, as the online form requested information I could not provide.
Once your SF application has been approved and your evidence sent off, start looking for a part time job. Your student loan + grant won't be enough for you to live very well on its own. You'll need a part time income to top it up.
Hope that helped. If not, ignore my rambling. X