Unfortunately, unmarried women and their children suffer an appalling lack of rights and protections wrt relationship breakdown, inheritance tax, entitlements to pensions, property, etc. They do not even have the rights of lodgers or tenants should they not own the family home and are pretty much up shit creek, with their children, if the partner dies. If he gets hit by a bus, she won't even be able to get a fiver from his bank account for supper. If he sells her the house foe a pound, then dies within 7 years, the gov't will still claim inheritance tax as that is seen as a tax dodge. There is actually no such thing as common law in this country.
If you do not marry, please ensure you figure out other means to protect yourself. Does he have life assurance, etc. Check the pensions and get him to add you as the beneficiary (if he legally can). Get him to sell the house to you or put it in the children's names ASAP. Sit down and explain this all to him and ask him whether he wants his children to suffer if the worst happens. You do not need to be going through legal hell while grieving and supporting your children through the loss.
Book the registrar and have a serious talk with your partner.
I went through this and am now married.
I'm really NOT into marriage, but it was necessary for my protection and that of my children.
The rest of us, we must fight like hell to make sure the government gives unmarried mothers living with their partners more rights and protections. Because right now, they have none.
Good luck.