Some of the things you mention can be dealt with, one by one: make wills, take out Lasting Powers of Attorney, take out life insurance naming each other as beneficiaries, make sure any property you own is in joint names,etc. Getting married covers some but not all of these.
Feeling "no need" to get married is rather short-sighted, as one of the purposes of marriage is to put these practical measures in place and give more security to couples. If you're going to tackle each of the items separately, you might as well marry.
Widow's pensions and benefits are usually only for people under retirement age, but they're payable their a spouse, not to anyone else. Occupational pensions sometimes provide for the spouse of deceased pensioner (e.g. half of the pension, for the life of the surviving spouse) but if you're not a spouse, you don't qualify. Some element of a deceased's State pension may be passed to a spouse in certain cases, but if you're not a spouse, you don't get it.
Most people who are committed to each other for life demonstrate this by getting married. That's what marriage is there for. It's your choice.