Would it be necessary to have all original beneficiaries under the ROI sign a DOV for an intestate estate even where not all beneficiaries are impacted by the deed and no additional inheritance tax would result from the deed.
No, just the impacted beneficiaries and administrator(s).
I had neglected to point out that the administrators (2 of them) are also the only two beneficiaries.
Presumably then both administrators (and therefore both beneficiaries) would still have to sign even if it does not impact the IHT position?
Best practice is for both to sign, but if a beneficiary is giving up their entitlement, then I can’t see a necessity for both administrators to sign. However, I cant back this view up with the relevant chapter and verse.
An Instrument of Variation (to give it its correct name) is a taxation fiction. In the real world, the Original Beneficiary is giving part, or all, of their entitlement to a Donee. IHTA requires the IoV to be in writing and to contain the relevant election(s). It only needs to be signed by the OB unless additional IHT becomes payable, in which case the PRs must also sign.
If the PRs are not party to the IoV, they must account to the OB for their entitlement and the OB then gives to the Donee. However, of the PRs are party to the IoV then they are bound by the revised distribution and must account directly to the Donee.
It is standard practice to do an IoV by Deed. The choice of parties will depend on whether the OB wants their co-PR to know about the gift they are making or not.