Once you loan is formally approved, an approval letter is sent to your solicitor.You solicitor will then contact you and ask if you are happy to proceed with the purchase. On your approval they then send the approval to the vendors solicitor, who exchange contract.
This is called unconditional exchange and once this occurs, you are obligated to proceed with the purchase. If you decide that you no longer want to proceed after this point, the vendor is entitled to your holding deposit.