Go and see a solicitor for a free half hour. I would phone Women's Aid first and see if they can point you to someone who will run through the options for free - they should be able to, and will also be able to give you good free advice themselves!
Is divorce an absolute priority? As in, you need to be away from him and separated and you and the children protected from him... but there are other, non-costly ways to achieve that. You could maybe focus on that, and divorce itself could come later? - either through unreasonable behaviour or just after several years' separation.
The first thing is housing. You say you're separated but you're still in the same house - why? Do you own or rent? Is he refusing to move?
If you are renting, if I were you I would simply move - is that possible? Are you financially able to do that? If not, if money is a problem and/or you are HA, ask Womens' Aid/solicitor how you get him out. You should be able to - he is an abusive drunk whose habits are a danger to the children. You could apply for him to be taken off the tenancy, and the police could remove him.
If you own your house, it is more complicated... but again, think of the goal - to not have him around, causing things to be harder. Again, take advice - I think here you would have to divorce or be divorcing to force a house sale, but there are other ways... if you have little or no equity, the house isn't necessarily any more an asset than a rented property, and you could tell him that you either split and sell the house, or you'll move out anyway and leave him to it.
With regard to housing, the important thing is for you to start thinking solo - I bet that although you're separated, he's there and you're washing his socks and cooking, right? Stop all that - you and the kids are a unit, he has shown that he isn't and doesn't want to be part of it - he'd rather drink and take drugs. Don't think 'but where would he go?' Not your problem. And don't be paralysed by thinking 'Well if I don't make sure he's got somewhere to live etc., I'm never going to see maintenance off him...' - fact is, a bloke like this is unlikely ever to step up to his responsibilities, so think solo - get him out, get away from him, and expect nothing in the future.
If you can get away from him and live elsewhere with the kids, you are 99% there.
Other stuff. I would stop protecting him in any way. Report his drug use to the police. Talk to Womens' Aid/solicitor about contact - fact is, if you split and he decides to make your life difficult, he would probably do this through the children. Make sure you report his drug use as you need PROOF and a paper trail that he is a substance abuser, in order to make sure that contact is monitored and that you don't end up with him having them overnight. You should, once you are living separately, be able to get a residence order ( a legal doc saying the kids live with you, so if he decides to play silly buggers and not return them, the police will simply go and get them) and a contact order which sets out terms, and which you have asked for based on evidence of his alcoholism and substance abuse.
The result: you live in a house with the kids, without him, and his contact is controlled. And to do this, you don't have to divorce, you don't have to have even an argument with him, and there's nothing FOR him to 'fight all the way' against.
And if he starts harassing you once you are away - you get a restraining order.
So - start the ball rolling now. Go to WA and talk to them, about last night, and the danger the kids were in, and get advice on how to proceed. And get tough - he's no longer your concern, don't listen to threats or blackmail or tears - focus on getting OUT and AWAY from him.
Divorce can come later.
Good luck!