I owe about £800 to 2 different catalogue companies, and I am not paying them. Will this hurt my credit score?
It will only lower your credit score if it is reported to the credit bureaus. Not all lenders or catalogue companies report to the credit bureaus.
However, if you default on the account(s), and they are given to a collection agency to be collected, in all probability they will be reported to the credit bureaus and have an impact on your credit score.
Can you afford to pay the accounts? Can you afford to set-up a repayment plan?