Interest in Possession Trusts

[Tara]

I was reading about these Interest in Possession Trusts (IPT) on the UK Gov website. The example it gives was as follows.

“You create a trust for all the shares you owned.The terms of the trust say that when you die, the income from those shares go to your wife for the rest of her life. When she dies, the shares will pass to your children. Your wife is the income beneficiary and has an ‘interest in possession’ in the trust. She does not have a right to the shares themselves.”

My question is can such Trusts be varied according to the wishes of the settlor? For example, in the example given it says you create a trust for all the shares you owned. So can a settlor create an IPT for a half, or a quarter, of the shares that they own?

And also, does all of the income from the IPT need to go to the wife or named beneficiary every year? Can the settlor instruct that only a part of the income, or 30% of the income, or £50,000 per annum, goes to the wife or named beneficiary every year? And if this is allowed, would the remaining annual income go to the children every year?

And also, does the right to the income from an IPT always remain for life? Can the settlor instruct that the income for the wife or named beneficiary should end when the children have reached a certain age such as 18?