In most parts of the world common law marriage is recognised. I actually think it's a disgrace and backwards thinking that the UK simply won't recognise it.
I don't. What about a case like this, which I've just made up as an illustration?
Annie was married to Bob for 30 years and they had a daughter, Cassie.
Bob died. Annie inherited Bob's entire estate. Bob trusted her to pass it on to Cassie in due course, if there was anything left after providing for Annie's old age, but this was a verbal understanding between them, not written in the will.
Annie meets Dave. Dave moves into Annie's house. Dave has no property or capital and was previously renting.
A year later Annie dies very unexpectedly and without leaving a valid will. Under the intestacy rules, Dave gets nothing because he and Annie were not married. Everything goes to Cassie.
Under the rules you're suggesting, even though Annie had not married Dave, he would have had a claim on Annie's estate.
Is that what you think Annie would have wanted? Is that what Bob expected? Is that fair to Cassie?
I believe that people should make a conscious decision to enter into a contractual relationship. If you don't formalise it, the assumption should be that you didn't want to, probably with good reason.