By commercial arrangement, do you mean a proper loan agreement drawn up by a solicitor?
AND the rates are commercially realistic.
e.g. Take out a loan at 5% secured against a residential property and on-lend to a discretionary trust unsecured at 5% is NOT commercial.
e.g. on-lend at a 'commercial rate', but interest may be deferred indefinitely non-compounding is NOT commercial.
Why would you be entitled to a full tax deduction for your original loan in this case ?
It suggests partly another, perhaps private, purpose.
Cheers,
Rob