You can enter into an ISA with the government or with a private institution. You're paid a fixed amount, but what you repay is contingent on how much you earn afterward (source). 

The main difference between an ISA and a traditional loan is that loan repayment amounts are paid back in fixed sums, while ISA repayments can vary significantly.

Pretty simple stuff...unlike holiday gift wrapping. We still get nightmares from the ribbon fiasco of 2008.