He spends perhaps £30 - £40 per week on his habit
8-10 spliffs a day (63 a week?) doesn't cost anywhere like £30 - £40 OP!
If he smokes that much and says that's how much it costs, then he is TOTALLY bullshitting you! TOTALLY!
I used to be a stoner and have a couple of close friends who still are, and if he is smoking as much as you say, you're prob looking at £150-£200 a week. Easily.
I have a friend in a situation similar to yours. Her husband has smoked since he was 14 (he's 35 now) and smokes from when he gets up until he goes to bed. he has a PT min wage job, while she earns a good wage. Well on paper anyway, but after his habit, the car payments and the mortgage, they're pretty much skint. They have 2 kids, ages 1 and 4. Luckily her parents provide childcare. If they didn't, she would be totally fucked!
The kids don't cost much now but I don't have a clue what they are going to do when they get a bit older and start needing and asking for more stuff.
Like your DH, he gets edgy and angry when he doesn't have it. I think she just thinks it's not worth the hassle of forcing him to stop, but it's a real shame for her and the kids. I guess she justifies it to herself because he's "always been like that".
The only solution here for your husband is cold turkey. It won't be pleasant and he will need to find other things to keep his mind off it, exercise is always a good one.