If the overdraft on a director’s account with the company exceeds £10,000 it is treated as a loan. A taxable benefit will arise on the loan when the employee does not pay interest to the employer at HMRC’s official rate of interest. The cash benefit is the difference between interest calculated at HMRC’s official rate and the interest paid. The taxable benefit of interest calculated is required to be reported on form P11D. This will also be liable to Class 1A NICs

https://www.gov.uk/government/publications/rates-and-allowances-beneficial-loan-arrangements-hmrc-official-rates