The easiest way to do this and unwind the trust is for the trustees to appoint the trust assets in favour of the surviving spouse absolutely. This must be done within two years of death but not within the first three months following the death of the first spouse. Provided these time limits are complied with the appointment will be treated for IHT purposes as if the assets had simply been left to the surviving spouse outright, so the surviving spouse will have the benefit of the transferrable nil rate band. See Section 144 IHTA 1984. If the appointment is made within 3 months of the date of death it may be treated as a chargeable IHT event and there will be no reading back in to will.
Provided that the appointment is made during the period of administration the estate, which it normally would be, it will not give rise to a capital gains tax liability as the spouse will acquire as legatee.
The appointment out during the administration of the estate may be the preferred option but there are of course other uses for nil rate band discretionary trust wills, in addition to the preservation of the nil rate band of the first spouse, so this should not be done without full consideration of any other benefits preserving it might achieve.