My understanding is that specific authority is required to make a loan
to a beneficiary.
Where a loan is permitted, there will not usually be an IHT exit charge
if there has been no dilution of the trust fund, such as where the loan
is made on commercial terms or is repayable on demand.
If the trust doesn’t have an express power to make loans to beneficiaries, the trustees may be able to use their express / statutory powers of investment to justify a loan. Such powers would of course be subject to the standard investment criteria and the trustees would also need to consider their general duties to the trust and other discretionary beneficiaries. They should therefore, consider whether the loan should be made on commercial terms.
If the trustees conclude that a loan is possible, it would not constitute a distribution of capital as the benefit of the debt remains an asset within the trust.
I have a vague memory that an eminent silk (possibly before taking silk) once advised that the greater power includes the latter - ie if the trustee has power to distribute capital to a beneficiary then it would include power to lend. The memory is hazy so not entirely reliable.
Alternatively, a discretionary trust might provide the trustee with power to appoint the trust fund onto new trusts, identical save for an additional power to lend to beneficiaries. The exercise of the power could be phrased in these terms - inserting a new clause providing a dispositive power to lend.
I would agree with Naomi that an exercise of any power to lend (with or without interest) should not involve an exit charge.