In a nutshell, if you split up then there will be no compensation for the earning power, now and long term, that you lost in going part time to raise the kids. Not unless your partner is an absolute prince of a man, works this out and makes sure that you get it in the form of the existing assets and perhaps even from his own future earnings. You might get 50% of the house since you jointly own it but if you don't earn enough to buy him out/get anywhere new even if that asset gets liquidated, it might not do you a huge amount of good.
If he dies, there will be no IHT exemption for you (though the threshold on this is quite high, so it may not affect you; still, you don't know how much your house might be worth at the time one of you passes away). You will not get widows' pension (the recent ruling opened the possibility of that changing, but nothing has actually changed yet and it could go either way), nor automatic access to his bank accounts. If he's willed everything to you then you should get those when the estate is wound up but you are a lot more vulnerable to claims on his assets from blood relatives. If you were married, circumstances would have to be exceptional for relatives to successfully contest you for a share of assets.
If you split up, then as before: your contribution in childcare and long-term loss of earning power basically can't be legally enforced because your relationship does not exist in law. The point of divorce settlements is to recognise the contribution of the lower earning partner and to compensate them fairly for the long-term loss of earnings they have made for the sake of the family. Spousal maintenance is rare these days (though it does exist) but you ought to get more than 50% of everything to compensate for the fact that he's got untouched earning power and you've been stunted long term with regards to career advancement, pension provision etc. Going part time and being primarily responsible for children affect your long term earning power, and a judge will recognise this.
Sadly, he could conceivably change his will or empty his accounts without telling you.
Quite apart from all that, though, I would be wondering why I was good enough to shack up with, have his kids and take a financial hit to raise them, but not good enough to make a legal commitment. There have been a few of these threads recently so I hope not to reopen a very tired and bitter old argument, but I will say that this is how I feel, and I know it's how a lot of other women feel.
There is nothing wrong with a woman wanting legal protection for her contribution and recognition of her relationship. In most cases, it's the smart thing to do and protects the woman's interests. So if this is indeed what you want - and it is certainly what I wanted - don't let anyone make you feel that you're being somehow unreasonable or anti-feminist.