Initially you will have a benefit in kind, as a benefit charge arises in each year to 5 April, based on the difference between interest calculated at the offical rate and the amount actually paid to the company during the same year. Therefore because you will not pay the interest until a later tax year, the accrued interest cannot be taken into account when calculating the benefit in kind.
The benefit in kind should therefore be reported on form P11D for each tax year unti the year in which the loan is repaid and the accrued interest charge is settled.
Once the interest has been paid, a retrospective claim for relief can be made, providing that you can demonstrate that a legal obligation to pay the interest existed. It is not sufficient to simply volunteer to pay interest in order to try and avoid a benefit.