Most UK credit cards use a variable APR, meaning the lender can change your interest rate at their discretion.
Base Rate Changes
The most common reason for an APR change is a movement in the Bank of England base rate. Lenders typically pass these increases directly on to customers.
Risk-Based Changes
Lenders constantly monitor your credit file. If your credit score drops significantly, or you take on large amounts of new debt elsewhere, your lender may increase your APR because they now view you as a higher risk.