My solicitor told me that that if you are sure the marriage is dead, there is no advantage to separation. It is best to just divorce and get the best possible settlement. Be prepared though that if he fights, it won't be cheap.
However, I did get practically everything as the Judge was not impressed with his behaviour and lack of respect to the legal process. My ex thought he knew best, didn't have to comply with deadlines, turn up to Court etc, etc.
You do need to be able to hold your nerve, but it was worth it for me on so many levels, not least of which was having the Judge serve my abusive ex his arse on a plate.
I would recommend you work backwards financially to see how much you need without working first, including housing, living expenses for you and children, and how much pension fund you need (probably somewhere between £500-750,000 on current figures). Once you have that, plug a realistic salary in and see what that gives.
In the meantime, whilst you are filing, put in a claim for maintenance pending suit, so he has to continue current financial commitments. It will take the pressure off you, but will also give you an idea of how he is going to act going forward.
As a pp said, secure all the assets you can. Move everything from joint savings accounts, so the money is secure. You need to be absolutely ruthless once you start, put yourself first because he certainly will only looking after his own interests.