Op do you/did you own a property jointly during those 8 yrs?
What is the status of that property now?
How is he planing on funding his deposit on his new house?
What was his reasoning for changing his name by deed poll?
I think you can see that his lifestyle will improve if and when he and his girlfriend marry and buy together, given she already has 2 properties and i would assume equity in both at the least. Any investment she makes , i am sure , she will protect. He is changing his name so clearly unusual behaviour. I wonder if what you are asking is, if he and her build a financially comfortable life together , have a nice expensive home etc can you benefit from that if you are-still legally marred. The answer is no. You and he will only share any assets accumulated during your marriage /relationship and he will need to pay maint for your 2 shared children. He will be able to clearly demonstrate you were separated through council tax, where he was living addresses and banking . If you and he did not have or acquire much during your 8 yrs together then sadly you will only get a share of that.
Op, i mean this kindly, it is clear you have been hurt. See a solicitor, get divorced and a judge will formalise all your financials ( including any pension entitlements) for a clean break. Child Maint will also be documented and enforceable within that structure.
Start to rebuild your life . I think you also posted this exact thing last year. I read something very similar. You need to move quickly now to safeguard any/your joint assets as they currently stand. A jointly owed home will be yr biggest asset in terms of division. I suspect his new GF will ring fence all her assets before she buys with him / marries him. Her money may well springboard them but it will not help you.
Sorry OP. The guy is an arse and you will better off in all respects when you are free of him in terms of marriage and finances. The more time you put into hating /sabotaging him, the less you spend enjoying yourself and your children. Divorce him and move on. Staying married to him , in the hope of more money or to be disruptive, will only ultimately hurt you in the end.