OP, I think you're getting quite a lot of flack on here. The reason is because posters on Mumsnet get tired of saying the same thing over and over again to different people.
If you give up work to be a stay at home parent, or even go part-time, and are financially reliant on your partner, you are potentially making yourself very vulnerable. It means that you are contributing a lot of unpaid labour to the household, allowing your partner to carry on doing paid work. If you were married then you would be better protected in the event of your husband's death, and if you got divorced you would be entitled to a share of your husband's assets to reflect the unpaid contribution you have made. If you are not married then there are certain things you aren't entitled to (or aren't automatically entitled to) in the event of your partner's death, and you aren't entitled to anything if you split. The non-financial contribution you have made isn't recognised, because the law doesn't see you as anything more than flatmates with separate finances.
That said, if you are a low income family without a lot of assets, it may not make a huge amount of difference if you split. Divorcing a husband with assets of £500,000 means you get a share of £500,000. Divorcing a husband with £20 in the bank means you get bugger all. If your partner were to die, however, there are certain benefits you would only be entitled to if you were married, which would be very valuable to you on a low income.
Nothing you have said in your posts indicates that your partner doesn't want to marry you because he wants to protect his wealth. (Your posts don't seem to suggest that he actually has a lot of wealth to protect, although I could be wrong about that.) So if you would rather be married, you should sit down with him and point out all the ways in which your current situation makes you legally vulnerable, and say you would like to get married even if you just do a quick and dirty registry office wedding for £200.
Whether you get married or not, I think you should be making plans to re-enter the workforce. Even if it's not feasible until your older children are in school and your youngest is entitled to subsidised childcare. It's just not good to be financially dependent on a man, whether you are married to him or not.
Good luck.