When you apply for a loan or credit you have to complete a form, which lenders will use to work out your ‘credit score' based on your personal circumstances. They will also check your credit history with credit reference agencies, to see how much risk they're taking by lending you money. You can check your own credit file and have it corrected if it's wrong.
Experian and Equifax both offer a service where you can access your credit report to help you understand and manage your credit rating.
If you've been refused a loan or credit, or you're worried that you might be, you can check the information that's kept about you. If it's wrong you can ask for it to be corrected. If the information is correct and you're worried about your debts you can get free help.