I think that some of my points may have been covered by posters while I was writing this. Apologies for any duplication.
Lurk
You may well have answers to the following points. If so - genuinely - I'd be pleased to know:
Re occupancy after split with co-habiting partner: "If your ex-partner owns the family home in their name alone, you do not have an automatic legal right to remain there....if your name is on the mortgage, you’re liable for the whole debt, even if it’s a joint mortgage with others." www.moneyadviceservice.org.uk/en/articles/protecting-your-home-ownership-rights-during-separation
A registrar's office wedding can cost no more than around £200 in fees etc. Are you honestly saying that a decent solicitor would charge only that for drawing up and executing a formal pre-marriage agreement or marriage-based trust deed involving children? If so, let's hear the name of the firm.
Ad far as I know, anyone, married or unmarried can change a will unilaterally, without informing anyone else, whether their partner or their spouse. However, if they are married, their spouse has certain rights over their estate, whether or not the deceased left a will and whatever that will says: www.citizensadvice.org.uk/family/death-and-wills/wills/ These rights cannot be over-ridden. If a couple are not married, these rights do not automatically apply.
When anyone dies, married or not, their estate is liable for their debts. Not their unmarried partner or their spouse, though any debts will obviously be deducted from the sum they will eventually inherit. Simple explanation here: www.saga.co.uk/magazine/money/personal-finance/inheritance/what-happens-to-debt-when-someone-dies
Inheritance tax: It' s the value of the estate, not the house. Generally, the estate is liable for the tax if a spouse/civil partner does not inherit. There is a higher threshold before tax applies if the estate is left to the deceased's children.
Re trusts. Yes they can work but there are limitations and they are not cheap to set up. Nor are they suitable for all circumstances.www.thewillpractice.co.uk/property-trusts.html
What's all this - genuinely puzzled - about 'the courts' taking over? As many of use have said, there are clear rules about what happens to property during and after the break-up of a marriage, civil partnership or co-habitation. These rules are well-known and well established. It's sensible for anyone entering into a major financial comittment with anyone else to check them out and/or get proper legal advice.