I’m in a similar position to you and sought legal advice to make sure I was as protected as I can be. Here’s what she told me.
Owning a property as joint tenants
This is where you each own half, regardless of who pays what towards it. If you split, 50:50 would be the starting point for any negotiation. The age of the kids at time of split might be a factor in deciding who gets what, but generally you each would have half the house in a split.
If one of you died, you would not automatically inherit the other’s half like you would do if you were married. One way around this is to write wills specifying that you want to leave your half to your partner. However, if your share of the property is over the inheritance tax threshold, the surviving partner would have to pay inheritance tax, which they wouldn’t have to do if you were married.
Owning a property as tenants in common
This is where you each protect only what you put in. So if you pay 20% towards the house and your partner pays 80%, if you split you walk away with 20% of whatever the property is sold for.
Again, the same principle applies for inheritance. The partner does not automatically inherit the dead partner’s share. And if it is willed to them, they are liable for any inheritance tax.
Also, (and I’m not sure if this applies to 50:50 ownership too), if you own as tenants in common, technically you can each sell your share in the property to someone else. So, however unlikely it might seem, you or your partner could sell your/their share to a completely random stranger and the other partner wouldn’t be able to do anything about it. (I’m sure you could do something about it but it would be very drawn out and expensive.)
Pensions
If you’re not married you have no automatic right to each other’s pensions in the case of a split or one of you dies. If you’re married you have an automatic claim.
Some pensions you can name your partner as a beneficiary. Some you can’t.
Last thing to remember is that even if you have wills sorted, wills can be changed in secret. If your partner dies you could potentially find that after that huge row you had about him not putting the top back on the milk, he went and changed his will to leave everything to the Cats Protection League.
In summary
It’s possible to replicate most of the protections of marriage through joint/common tenancies, writing wills and naming beneficiaries on pensions. But these remedies are never watertight and basically it’s easier and cheaper (in terms of the actual marriage certificate vs solicitor fees) to just get married.