What is the max interest BTL company can pay for borrowing from the Director

Tax Planning and Financial

Hi there, the period under question is from 01 June 2021 to 31 May 2022. And the borrowing was unsecured with very high risk that the lender (in this case the Director) can loose all their money - point I am trying to make is that the rates from a third party wouldn't have been low. Any fair method to determine/ substantiate a reasonable rate of interest please, or rather any established market practice that anyone is aware of. For example, borrowing using a credit card product is a fairly similar i.e. unsecured and readily available but indeed with a high cost.