"he has significant savings that he keeps separate from our savings"
Hmm.
"Re doing it in a reg office- my only living sibling did this and told no one. My parents were absolutely heartbroken about this (and still are 10 years on)."
Your parents need to get a grip. It's not about them. You (will) have two children by this man, do they seriously expect the whole virgin-in-white schtick?
"Whenever I speak to him about it he says he wants to do it properly and propose etc."
OK, I wouldn't actually say this, but think it inside your head just to blow some steam off -
"Well the time for doing it your-idea-of-properly has now passed. We are now at the stage where we need to take care of practicalities, for the children. I do not want a proposal or a big fancy wedding, I want a marriage certificate, and I want to have this all done and dusted by March. Two witnesses and having the banns read is all we need. If you won't organise that, then I think I can safely assume you don't want to be married to me, and we need to talk about that. You first."
And breath.
"I have so much anger and resentment about the issue that if he ever did propose I'd know I'd just tell him to fuck off. On our wedding day I'd just feel bitter."
This is the important thing. It has the potential to affect your relationship, permanently. It's hard to respect someone when they spark resentment in you; and eventually, loss of respect leads to loss of love
.
I think this is what you need to have a conversation about. How this lackadaisical approach is making you feel. Tell him about the anger and resentment. Tell him it is affecting how you feel about your relationship. And tell him that his 'do it properly' notion is now a problem for you. He has had many opportunities to propose, hasn't taken any of them, and you need to know if you are planning a future as his wife, or just as his bidey-in. Because you need to make different financial arrangements for YOURSELF depending on those two circumstances.
He has pissed about for long enough.