OP, my hair is practically standing on end after reading all of this! I know you said you want to be married to him, but...if you think about it, isn't it more that you want to be married to the idealized version of him? I screwed up badly when I was younger and married a guy who acted just like your fiance, down to draining savings, not paying his fair share of kid expenses and expecting it to come out of my budget, everything. If you marry this man, this is what's likely to happen:
*He is NOT going to get any better about spending and budgeting, not for any length of time. He'll probably buckle down long enough to lull you into a false sense of security until you agree to let him handle the accounts - and I know you'll say you'll never let him handle that again but you will. He'll be super responsible for as long as it takes and put on a charm offensive and then as soon as he has control of the money again all bets are off.
*He will continue to be selfish and entitled. His wants will always come before your and your DCs needs on the grounds that he Works Hard And Deserves These Little Luxuries, and really, they're SUCH little luxuries, you don't honestly think you could deny him this little thing, do you? Only it's not one little thing every so often. It's little things all the time, the kind of little things that fritter away the £10K that was meant to go to a home for himself, you and his own children. The kind of Chinese trinkets and gadgets from eBay that are more important than a £200 refrigerator for his family, because HE wants them.
*When you put your foot down and say NO, that won't even be close to the end of it. He will wheedle. He will sulk. If he's anything like my ex (and he sounds VERY like him) he'll even surreptitiously break or damage something of YOURS - since he's not allowed to have "nice things" neither are you. He likely won't go so far as to be physically aggressive because he doesn't want to risk losing an easy source of income if you were to leave.
*While you are married to him, you're never going to have enough money. You're going to have to argue and scrimp and negotiate for every single thing you need to spend money on for yourself and the kids, and probably the home as well. And when outgoings exceed income, it's always going to have been YOUR fault, for all those fripperies like CLOTHING for your growing LOs.
*He's going to get the two of you in financial trouble and to avoid bailiffs or utility cutoffs he's going to push you to try and touch your family and friends for "loans", and if you refuse he may go behind your back and do it. You'll owe people money and somehow there'll never be enough free in the bank to pay them back. He will have a million excuses as to why he can't ask HIS family, of course.
OP, I know this sounds like I'm doomslinging all over the place, but I promise you, I know this type of man and I know how it works. I was stuck for nearly ten years and when my DC and I finally got out we had next to nothing and my credit rating was annihilated. Please don't put yourself through this. DO NOT legally yoke yourself to this man - get out now while it's a matter of a few lost deposits and some blushes over the wedding cancellation emails and phone calls, rather than a long drawn out adversarial process that ends up costing thousands of pounds and ruins you in the process. 