You have unfortunately learnt the hard way not to loan money to current friends, who may let you down when the time comes to repay, though it is not an unusual position and happens to many people. It seems as though they have acknowledged the debt and made offers to repay and this could be important, as the usual response is that no debt exists. As others have said go and see a solicitor with all the evidence you have of the debt and the borrower’s ability to repay, ask them to write on your behalf and make a reasonable (it is important in law to show you have behaved reasonably) repayment suggestion, they may just pay you on receipt of the letter.
Put a time limit for acceptance of your terms which if not received you will instruct your solicitor to take action through the county court, if no acceptance start proceeding to recover the sum. Your solicitor will guide you and explain it may be a long process and the court will not recover the money, make sure they have the ability to pay before any action.