Ignore debt collector they have no powers OP. Google everything they send you. Reference numbers eg "form DB57" or whatever may be totally meaningless and they've made it up to frighten you into paying. Don't trust them, all they want is to bully someone into paying the debt, they don't care who.
Don't answer door to them. Definitely don't phone them, they'll use the opportunity to manipulate and extract information from you. Write to them. Don't give any information they don't already have, such as your real address (I imagine it's in your name at your mum's address) or email address or phone number or banking details. Tell them that the debt isn't yours, you've been a victim of fraud, the debt is your mother's, provide her name and address. Keep a copy of the letter. Send it "signed for" at the post office so you can see it's arrived and have a signature of who signed for it, take a screenshot of the signature page.
The only thing you mustn't ignore is if you get a Notice Before Action, because that means they're taking you to court and if you don't attend to fight it then you'll lose by default. If you do receive this then provide the evidence you need to provide to dispute the claim as per the forms. Don't be giving the debt collector all your personal financial info, expenses and income etc though, which they'll ask for on official-looking forms which aren't actually official and you don't need to provide this information to them. That's information you'd provide to the court if you were asking to settle the debt in installments due to financial hardship.
This is unlikely to happen. If you're in England or Wales (sorry IDK about the rest of UK) debts expire 6yrs after the last payment/admission to the debt by the debtor etc. A debt from 2016 they've no chance of winning in court, they have no grounds to take to court, it expired in 2022. If anyone makes payment towards the debt, however small or admits the debt is theirs, the 6yrs starts again. I imagine this is why it's fallen off your credit file, unless you had it removed?
Report to police, it's a crime and if you don't report someone, somewhere could maybe deem you to have accepted the debt. We have a duty to report crime especially where someone else like a bank may lose out.
Change any password your mum may have access to. If there's any chance your mum has ever had access to your bank or card details, contact your bank about the fraud too. If you still officially live with your mum when not at university, move out, she's not someone you can live with any more. Even if that means leaving university and getting a job to afford it.
When you sign up for the electoral roll at your address, tick the box to go onto the "private register". It means Joe Blogs Scammers Company Ltd can't search the electoral roll to find out where you live.