Also, if you aren’t married, there are things that you can do short of that, if you wish, that mean that the situation is not as black and white as some paint here. This includes wills, pensions (most will allow nomination of a cohabitee) and purchasing property in the manner that best reflects the relationship and input.
That's an incredibly middle-class perspective. All the numbers say that most people don't do that; aside from anything else, to do those things properly is actually more expensive than just getting married.
For most married couples, dying intestate, for all the complications, does "the right thing". You can have wills, and it might simplify matters, and it might be necessary for complex situations, but intestacy works; when you see shock horror stories about 60% of deaths being intestate, what it leaves out is that in the vast majority of those, the will would have made no substantive difference to what happened, and what happened was actually "the right thing".
The same applies, mutatis mutandis, to parental rights, powers of attorney, pensions, agreements about communal and individual property. People will claim that you needs powers of attorney even when married and, sometimes, you do: sale of joint property in cases of incapacity, notably. But in reality, doctors will grant spouses wide latitude in "best interests" decisions even in the absence of a PoA provided the family is reasonably united, as often will financial institutions ditto. And in the limit, it's a lot easier to obtain a court of protection deputyship if you're a spouse than if you're a co-habitee.
Of course, instead of being married for £200, you can sort these issues out one at a time, paying your solicitor several times that £200 for each. But marrying gets a bundle of sensible defaults. Even if you then have to override one of those defaults with a more complex document, you're still almost certainly in profit; you'd have needed a document at least as complex to do the same job unmarried.
That's before considering that separating as a married couple grants you access to divorce law which, messy and imperfect as it is, at least has the power to sort things out; messy, unmarried separations are just that, unless you want to sue each other.
I have an entirely unsentimental view of marriage: I got married because it didn't matter to my relationship so long as it was working, but would make life a lot easier if we separated or one of us died or was incapacitated, particularly with children. It's like contracts: business deals which are going well don't need contracts, you only need to revert to the contract when you fall out with the counter-party or the deal goes off the rails. Then you're bloody glad of a decent contract, drawn up by decent lawyers. Or, alternatively, a boiler plate marriage.