A gift from one spouse to another or from one party in a Civil partnership to the other is Inheritance Tax free So no tax on the first death.
If the first to die does leave everything to their spouse or partner then on the second death the surviving spouse or partner can leave £650,000 tax free
Otherwise each of us can only leave £325,000 tax free and everything over that is taxed as 40%
Most Inheritance Tax planning needs to be done during one’s lifetime.
There are sometimes steps that can be taken to reduce the tax liability even after someone has died but these are limited and are only available to a limited number of people..
It's a very complicated area of law and very dependant on your personal circumstances and the assets you own