I'm in a similarish situation OP. Except DP and I always said we'd never get married and I've done a complete 180 since we've had our DD and now really want to be married. He still doesn't.
I said if he wouldn't marry me then I wanted to go to a solicitor and get legal advice, which he was fine with.
The solicitor said it was much easier to tie everything up legally by getting married!! But the same protection does exist in common law and statute, it's just piecemeal and more expensive and difficult to access if you need it.
Here's what she told me:
Own property as joint tenants, that way the rule of survivorship applies if one of you dies.
Make sure our wills now leave everything to each other and, if estate is over the inheritance tax threshold, the difference is put in trust for our children and we are trustees.
On the issue of separation, she said that that's where you're most vulnerable. Because if you give up work and your earning potential to look after the kids, normally that hit to your career would be recognised in any divorce settlement. What she suggested was, if I was ever in a position where I was going to make myself completely financially dependent on DP, we should draw up a cohabitation agreement which says how much of our savings, assets, etc, I should get if we split up. It's not legally watertight but it does show a court the spirit and intention of the partnership when the agreement was entered into in case things get messy later on.
There are other things you can to like getting lasting power of attorney to be next of kin if one of you becomes incapacitated, etc. But that's quite fiddly and expensive.
Anyway, all that being said, is your DP willing to share everything with you in this way? What was crucial for me was that my DP is happy to go 50:50 on everything, he's even happy for me to change my name to his if that's what I want to do. He just doesn't want 'the government getting involved in our relationship'. Fine. whatever.
But if he wasn't willing to do all of the above, I'd be questioning whether he was serious about a long term commitment.