Shakespeare wrote, "Neither a borrower nor a lender be (For loan oft loses both itself and friend)" and that's never so true as when loaning money to friends and family.
You could open a court claim for the money (see this:
https://www.gov.uk/make-court-claim-for-money/overview ) but unless you have some kind of real proof the money you gave your friend was a loan and not a gift, it will be up to the judge whether or not you prevail. And even if you do, it might not be enforceable if your friend is a minor and/or has no attachable assets.
I guess the bottom line is, you are not going to be guaranteed repayment of your loan unless your friend decides to make good on it. Chalk it up as a learning experience and be careful in the future to whom you lend your money.