There are a lot of strong opinions on this thread.
Ultimately OP you have made decisions which mean that in the event your partner leaves you for any reason you will be financially in a very difficult position.
You need to have some open and direct conversations with your DP about how to bring more parity to the financial situation so that if he leaves, gets I'll and can't work or dies you are not left destitute.
If your children are now all in school then you can work, that would give you more security, you may not want to work, you may find that things like after school clubs need to reduce to allow for it but life is like that.
I worked full time from when DD was 9 months old, DH also works full time. We're in a slightly different situation in that I am the higher earner, with more than 4 times his income, so if we split I would be the one paying support to DH, not the other way round.
Marriage is sensible, it can be as simple as going to the registry office, it doesn't need to be a huge shindig.
If not a marriage then you need to ensure you have clear wills that define what will happen with any assets including pensions, and you need to have complete transparency and access to joint funds. You also need to check your Government Gateway account to make sure your NiI contributions are up to date so you get your state pension when it's due, and it would benefit you to have a private pension too which your DH should be willing to set up.