A credit file is the record of a person's borrowing and repayment history held by credit reference agencies. Lenders read it to decide whether to offer credit, and in the UK it is maintained by Experian, Equifax and TransUnion.
In one line: A credit file is the record of your borrowing history that lenders use to assess applications.
How a credit file works
The file lists credit accounts, balances, payment history, defaults, any court judgments and the electoral roll entry. Lenders report updates to the agencies, which build the picture used in lending decisions.
Applying for a 2,000 GBP loan, the lender checks the file and any linked score to judge risk. A history of payments made on time generally reads more favourably than missed payments or defaults.
Each agency can hold slightly different data, so the same person may see different scores across Experian, Equifax and TransUnion depending on what each lender reports to them.
Credit file vs credit score
The credit file is the underlying record of accounts and history. A credit score is a number an agency or lender calculates from that data, and there is no single official UK score.
Lenders apply their own scoring to the file rather than relying on the consumer-facing number, which is why an application can be declined despite a high advertised score.
Primary source: Information Commissioner's Office: credit