If so, then how? Can creditors report defaults directly to credit rating agencies? What is required for them to be believed if they don't have a CCJ to back them?
Yes, such debts can harm credit ratings
Creditors can, and routinely do, report defaults, and late payments (more than 30 days late) that are not defaults, to credit reporting agencies (CRAs). A consumer can issue a statement of dispute to the CRA, under the US Fair Credit Reporting Act. The CRA will ask the (alleged) creditor to confirm the validity of the debt. If the creditor confirms its validity, the CEA may continue to list the debt, but must include in any report that the debtor disputes the debt, and a statement of the dispute.
A consumer may bring an action against a creditor for a false report, and can get an order to stop reporting the debt if the case is won.
Any potential creditor may obtain a report from a CRA and take items listed into account except when a specific provision of law prevents this.