@Iwasneveragoddess I feel for you!
My stepson has never lived with me. I met his dad when he was 16, and we didn’t marry and move in together until he was mid way through his first year. He’s a great man, I’m proud to have him in my family. But as I say - he’s never lived with me, and doesn’t now either. He rents full time at uni, and if he does visit home, home is his dad’s old house (currently inhabited by older child and friend). Although I care about my stepson a great deal, we’re not that close. If I were to split up with his dad, I wouldn’t see stepson for dust! That’s not saying anything mean about him - we just don’t have a close relationship, as I came late into his life.
Of course, student loans can’t be based on how much we like each other!
But it does seem ridiculous that it’s based on my income when we have never even ever lived in the same household! 
I earn 4x his dad’s salary, and it is me who is making up the difference. Which I don’t resent in the slightest - I knew this when I married his dad.
But it has caused an issue for my stepson. He REALLY hates taking money from me. Simply, because he’s a lovely man who thinks it’s unfair. Of course I don’t give him money - but he’s not stupid. He knows his dad’s income. So although the £300 comes from dad’s account, he knows damn well that dad isn’t contributing to our bills, to be able to afford to do that. I chose it, I don’t mind. But he does. As a result, he wants to work more hours and not take his dad’s top up. Which is honourable but so annoying!
I would agree with the household assessment for a grant, but it seems unfair for a loan. I guess the issue is that so many won’t have to pay back the loan - so it actually in that case is more of a grant. So I do get the public policy decision.