Don't bother if you don't want to. From reading posts on Mumsnet, it appears marriage is a lifestyle choice that may give access to certain benefits in the event of one spouse's untimely death or, if one party to the marriage is much richer than the other, will give the poorer spouse access to more of the other person's wealth in the event of a divorce. Neither sound like very convincing reasons to me.
I was with my XDP for many years, we had children but I could never see the point of marrying so we didn't. Some pressure from his family, especially when we had the children, but we just ignored it. When we met, I had my own property so when we bought houses together I was on the deeds, they were half mine, so when we split up, I had no particular problems. I got more than half the value of our last house together as the children were going to live with me. XDP lied about his money, job prospects etc. when we did mediation, but if we had been married, he would just have lied more creatively and might have spent more time and effort hiding money before the split, although there wasn't much to hide. I got some of his squirrelled away cash because he realised it was reasonable that I should.
We decided our relationship was over, our split was broadly amicable and it was all very civilised. I would have hated to be like the lady in the news last year whose marriage was over but she couldn't divorce because her husband wouldn't agree. I also had a friend who had to stay officially married for ages after her husband abandoned her just because he wouldn't co-operate with the divorce proceedings.
We had many very happy years together, and I can't say that we would have stayed together if we had mumbled some words in front of a priest/council employee years before.
So if you don't want to marry, don't. It makes very little difference to anything.