Legal background to trusts and estates: parol trust
A parol trust (more commonly called an oral trust) is one created verbally by a settlor. It can be as binding as a written trust.
If it does not come into being until after the death of the settlor, it cannot be a will trust. It exists outside of the will.
This is subject to Section 53 Law of Property Act 1925 where land is involved.
Send all such cases to HMRC Trusts & Estates Technical Edinburgh.