outsider wrote: ..... Yes i'm aware of the two NRB's and your calc's are just about correct. The house equates to approx a 1/5 of the total estate.
She doesn't want to downsize any time soon, if ever.
So, if I understand you correctly, your mother’s total estate is about £1.1m and the house is valued at about 20% of that total, about £220k.
Hence assets other than the house account for 80% or about £880k and mitigation/extinction of liability to IHT would address reducing the value of other assets assessable to IHT by about £450k so as to lower total estate assessable to IHT from about £1.1m to £650k. As you wrote that the removal of the house would achieve this, I did have the impression that the house accounted for the majority of the value of her estate but it does not. So downsizing and gifting has less effect on mitigation of liability to IHT even if she would be willing to do so (which you write she does not).
But estate strategy planning for your mother should be conducted in a holistic way – her wishes, age, health, life expectancy, pension and investment income, value and nature of assets, etc.
In essence to mitigate the liability to IHT on her death, if that is what she wants to do, she can either remove assets from her estate or exchange assets into assets which attract relief/exemption from IHT (e.g. farmland, woodland, AIM listed shares, shares in unquoted trading companies etc).
My suggestion is for your mother (and you and/or your siblings at her invitation) to discuss her wishes with a specialist lawyer and financial planner to be reflected in her will and agree the lifetime plan for her assets and her lifetime income.