Inheritance Tax is paid by the estate of the deceased. As the house has been left by one spouse to another spouse, the transfer is exempt from IHT. As the late wife does not appear to have used any of her £325,000 nil-rate IHT band, that can be transferred to the husband's estate when he dies, potentially doubling his nil-rate band to £650,000. In addition, when a property is left to a direct descendant there is an additional allowance which you can read about here.
If the house is sold by the surviving husband to cover care home fees, then it won't form part of his estate. However, there may be some cash left in his bank which will count towards the IHT threshold and that will depend on how long he lives after selling the house.
I hope this helps but let me know if you have any further questions.
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