I'm sorry, OP, but your OH sounds like a bit of a dick.
Plant in the loft? Any passing police helicopter with infrared may take note of the hotspot. One plant? If he gets through more than an eighth a week, he's going to need more than one plant, probably on rotation so he's cropping frequently. Initial setup will cost a few hundred quid (does he have that spare?), electricity bill will spike, there's a danger of the electrics catching fire if he doesn't know what he's doing, and it will stink. Also, plants need daily attention - if he's growing, he's never going to be able to go away on holiday, or even just go elsewhere for the weekend. He'll be tied to the house. Is that what he wants?
Your priority when you have kids, as I'm sure you know, OP, is keeping a safe and secure roof over their heads. How exactly does your OH plan to provide this from a prison cell? He is no use whatsoever to his children if he gets caught. He can't guarantee he won't be caught. He says he wouldn't put you or the kids at risk, but the simple fact of the plant's existence in your house is what puts you at risk; he cannot protect you from that except by not growing in the house. Paint him a picture of how your lives would look, with him in prison and you and the kids evicted, and he probably won't like it.
It's this line that worries me though: he's told me he'll continue to buy off the street and spend our money on it if I don't agree. The two of you disagree on how to spend family money, and he utterly overrules you. Who the fuck does he think he is, to lay down the law like that? It's your home just as much as his. You decide together what goes on in it. If you don't want the risk, then he needs to accept that. Or else he can fuck off and get his own flat to grow in.