Low credit scores and a bad credit score are two very different things. The challenge is to ensure that you apply to a lender whose eligibility criteria you can meet. Many high street banks rely heavily on generic credit scores, whereas niche lenders are often more interested in your general financial conduct.
A low score usually means that you haven't ever taken out credit borrowing or haven't registered on the electoral roll.
Bad credit means that you haven't kept up with your debt repayments at some point. If you're not sure of your credit rating, it's very wise to access a copy of your credit file and ensure it is all up to date and accurate.
Credit reports show:
All of your accounts with credit borrowing, from mortgages to mobile phone contracts.
Any recorded court action taken against you, such as a CCJ.
Searches run on your credit file.