I'm in my first year of university and I've decided I'm going to leave. I've taken student loans etc... How does it work with them?
Once I get a job over 15k per year, will they automatically take what is owed to them? Also, do I even have to tell them I have left, because surely if I just do not sign the Student Declaration for my second year, they will simply assume I've quit?
You will need to tell them you've left, as if you decide in later years that you want to return to university it could complicate matters hugely.
If there are any overpayments in your funding (as it's the end of a semester there isn't likely to be much, if any at all) it will become immediately repayable, and SF will usually work out a payment plan for a set X amount per month. The rest will be taken on the normal terms (earning over 15k etc).