I have been through this process.
My parents had changed the ownership of their house to tenants in common and in their wills each left their half of the house to us children with a life interest for the surviving spouse.
In each will, the executors and the trustees were the surviving spouse and us children.
My father passed away two years ago.
The estate went through probate (it took four months), no inheritance tax etc.
Once the estate has been administered and half of the house passes to the trust, you then have two years in which to register the trust with the government Trust Registration Service.
It's free to do and quite straightforward. There is a lot of information you need to supply but it is all basic stuff like, names, addresses, NI numbers etc for everyone involved (the person who died, the trustees and the beneficiaries).
"I’ve looked into lifetime interest trusts but can see that there are some valid disadvantages to the surviving partner and costly to draw up with ongoing admin fees."
That's interesting, what do you think the disadvantages are? From my family's own experience there has not been any disadvantage and I can't see any disadvantage in the future either.
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"...and costly to draw up with ongoing admin fees."
I'm a little confused here. Yes it is more expensive but, I think the cost is generally reasonable.
For example, I just had a quick look at the current prices for the will writing firm that my parents used back in 2015. I don't know what the cost was back then but the cost at the moment is:
Single Will £250
Single Will with Trust £450
Double Will £450
Double Will with Trust £750
"...with ongoing admin fees."
We haven't had any ongoing fees at all. The trust doesn't come into existence until the person dies. It is an Immediate Post Death Interest in Possession Trust.
For us, it has been straightforward to manage ourselves and we have not needed professional advice.
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"We are looking to draw up mirror wills and want to include a clause that should either of us remarry after the other has died, then half of the total assets goes to our child."
A mirror will is not binding in any way on the surviving spouse. If your husband were to die first and left everything to you then you could take a young lover and leave everything to him (or the local cat's home charity) and nothing to your child.
A life interest can be for the life of the surviving spouse or until some other event. For example, there was a thread here recently where the OP's mother had an interest in the house until she either remarried or wanted to move out.
So, it is quite straightforward to write a will that says after your husband dies, half of the house is left in trust to your child and you have the right to occupy the house (or sell the house and buy another one) until you either die or remarry.