Dropping out of masters. Could I still reapply in the future?

Yes you can drop out now without having to pay anything. Ask the uni if you can defer the year of entry to next year if you want to go uni next year. I'm not sure if you can defer to longer than a year so you'd have to ask. If they say no then you just have to cancel your application if you want to go after 2 years or more and apply for a refund then reapply when you want to go. If you've applied to student finance then cancel your application.

So, I've just received my first payment installment from student finance. I'm hoping to try again next year.

If you've received a payment I'm not sure how much student finance you'd get if you apply again. I think you'd get the same amount minus the amount you've received now. I'm not sure how it works I've never dropped out after receiving a payment. Maybe call student finance (early in the morning is best to get through before 10) and tell them you've received a payment but you're deferring and ask what you're options are, how much student finance you can get if any. If you can't return the money and start again then I'd keep the money cos you'll need it to pay tuition fees when you do go to university or if they won't pay you anything then you'll need to use the gap year to figure out how you'll get the funds, maybe have to get a job if family can't help.