Deducting Inheritance from a Deed of Variation from an earlier Gift

[JG45]

A few months after my father passed, my mother gifted me £20k. A few months later, my mother decided to carry out a deed of variation on my father's will so his savings of £12k would be left directly to me. For the purposes of the IHT nil rate band/HMRC, would it be possible to simply deduct the £12k from the original £20k received from my mother previously, so that gift of £20k would actually now be considered as a gift of £8k and that the remaining £12k is considered as coming directly from my father as my inheritance from him, due to the deed or variation. Or will my mother need to transfer the £12k from my father's Estate after the date of the deed of variation being carried out?

We would prefer to deduct the £12k from the original £20k gift but naturally wish to carry out things correctly in relation to HMRC and nil rate IHT bands.

I should also add that my parents' total estate will be substantially below the current total nil rate band for married couples (well below half the current £1m threshold) so unless the nil rate bands are massively reduced in the coming years, there will be no IHT to pay after my mother's passing so our situation is not related to trying to avoid IHT, it is more about helping me financially in the present.

Many thanks in advance for your help.