Hi Susan Wong
Thank you for your question.
The declaration of trust does need to be a written document.
The following guidance gives further information.
Ownership and income tax
I would also refer you to the following guidance if you are only transferring the income to your wife as the settlements legislation may apply.
Settlements legislation
A declaration of trust is a legally binding document that creates a trust.
Most types of trusts giving an interest in property to somebody who is not the legal owner now need to be registered with HMRC via the Trust Registration Service.
You can find guidance on the registration requirements and how to register here :
Register a trust as a trustee
There’s also more information about what type of trusts need to be registered at :
Types of trust that need to be registered