DP and I have been together nearly 20 years and have kids. Neither of us wanted to get married. In my experience, marriage doesn't per se keep couples together, and if it does then, as the offspring of parents who stayed together 'for the sake of the children', that is sadder still, imo. I also very much didn't want to take my place in the long historical tradition of women being married off by men to other men for the purposes of sealing deals of various sorts. DP would happily have got married if I'd wanted to, but completely understood and respected my position.
We have a jointly owned home and mortgage, joint finances, and a joint share in a business as well as a business each of our own. DP is named on the kids' birth certificates and in parental responsibility agreements (no longer relevant). We have life insurances written into trust for each other and naming the kids as second tier beneficiaries if we should die at the same time, negating inheritance tax concerns. The only expense any of this incurred was a nominal notarising fee - a marriage licence would have been more - and none of it was rocket science to figure out.
A lot of bollocks is talked about consent in medical situations. NOK is an administrative concept in this context, not a legal one. The reality is that no one, whether a spouse, parent or anything else, is entitled to make medical decisions on behalf of anyone else. In the event that a patient can't make their own choices/decisions, a medical team will act in the best interest of the patient. Family views would obviously be considered, to the extent that that is (medically) appropriate. Married versus unmarried partners would not be distinguished between in this situation. If you want to be sure that a doctor will discuss your medical care with someone else (i.e. not refuse on the grounds of confidentiality), just sign a letter giving that consent, have it placed on your GP notes and carry a copy in your wallet. That applies equally even if you're married.
If we could have had a civil partnership, we probably would have. That said, I was married previously (I had no choice, as we came from different countries), and when we split up, a bad situation was made much worse by the fact of having to go through the process of legally justifying our separation, and having the details form part of a (theoretically) public record. I agree with previous posters who feel that a relationship should be essentially a private arrangement. Although I can see that there may be situations where marriage affords particular legal protections, I am glad I'm not in that position.