Yes, I suggest the appointment of the life interest should specify that the entitlement includes the pre-appointment income, if it intended to mop up all of the income. If not, then the pre-appointment income may still be subject to the exercise of the trustees’ discretion.
Whilst, as Simon says, the executors will only be liable for basic rate tax on income they receive, the way it is taxed in the trustees’ hands will depend on whether distributions are made before or after the appointment.
When an executor makes any distribution, for income tax purposes it is deemed to include the estate income received up to that date. If the trustees have not appointed onto life interest trusts by then, the income element will be taxable in their hands at the trust rate, regardless of whether it is subsequently captured within the life tenant’s entitlement.
With any distribution after the date of the appointment, the income element will have been “franked” within the estate and any income tax adjustment will be reflected in the life tenant’s personal tax position.