"In UK, Wills trump a marriage certificate.
unmarried partners can Will their assets to each other.
If they live together, they can either be joint tenants of a rented home, or register their joint ownership at the Land Registry."
No they don't, as many mostly women find out
A will can be changed, many men don't make wills or don't update them, you can gift assets to a spouse tax free, an spouse can inherit the total assets tax free, for a non spouse anything left in the will is subject to IHT rules.
Marriage has 2 big advantages, you cannot disinherit a spouse, a spouse will get a share of marital assets including pensions, it works both ways if a woman is the main breadwinner, the man gets part of her pension. It does not include business assets unless it is joint business.
Of course you can protect your finances by wills and joint ownership but you have to keep close control not too difficult without children, with a family it gets a lot more complicated