You need to tread very carefully here.
You are the higher earner, you have a bigger pension. It's extremely unlikely you'd get 80% of the house, let alone 100%. Even if you agreed that between yourselves, it's unlikely a judge would sign it off.
Even though you think he won't see the children, you'll both be expected to house them appropriately. His mortgage raising capacity and income is lower than yours, so he could argue he needs more of the house equity than you.
If he's permanently housed with his new partner, that might work in your favour, though it's tricky and depends on several factors. Is he paying her rent? Does she own the house? Are there enough bedrooms for your shared children to stay?
Normally the courts prefer a clean break, but there might be a good argument here for a mesher.
This could allow you stay in the property until the youngest child is 18, at which point you sell and split the proceeds. (Starting at 50/50).
At that point you could downsize to a one-bed which would be easily affordable with half the equity.
They're waning in popularity, but might work here since he is housed with his partner (assuming there is room for the kids etc).
Pensions equality will be expected. So you won't be able to both keep your own - particularly if yours is substantially higher. You could give him more of the house to offset your larger pension. E.g. you get 40% of the equity and he gets 60% (depending on the numbers).
Marriage typically protects lower earners on divorce. And the courts won't allow something that is materially unfair.
You definitely need a lawyer (and a good one).
As PPs have said, start child maintenance proceedings. (But worth noting that this will reduce his salary further compared to yours, and increase his needs).
Also - don't be rushed by him. You want to do this properly and correctly. I'm not saying delay maliciously - that will just cost you both more money in legal fees. But you don't need to sprint through it all ASAP because he says so.