I've been thru this. Even tho you've been together for 20 years, it's only the 12 years of marriage that you can divide up. So, if he made payments on the house for 8 years before you were married, he gets the credit for that. Even if he was in a different property, which he sold and then paid a deposit on your current house, that's his money and it has to be included in the sums.
Regards the 12 years married, yes, that is all split up 50%. This includes the value of everything, which of course may not actually be hard cash. For eg. house value, cars, shares, other assets, pensions. So, work out what you are actually due and compare that to what you'd get if the house value element was carved up 75/25. You can let him stay in the house...he could raise a mortgage to buy you out from it, he doesn't have to actually move.
Re Pension, let's say yours has a current value of £150,000 and his has a value of £50,000, the total value is £200,000, so you'd have to give him £50,000. But, you say he was a big earner in the past, so maybe your Pensions won't be so different? But, if yours is much bigger, this may compensate you for the house value being divided 75/25.
You simply have to do your sums! If you do your sums and he would be getting unfairly a larger portion than you, you must employ a solicitor to draw up a separation agreement. This will cost about £1500. And he will have to get a Solicitor too, also at the same cost. Threat of this may make him re-think.
Also, who will get the Child Benefit? And will he be getting substantial Child Tax Credits? If his GF is living with him and she earns a lot, he shouldn't get any (if he's owned up to HMRC that she lives there).
I would not budge on anything. You are entitled to your fair share. If you don't get it now, you will regret it later, when you are mortgaged to the hilt and they are jetting off on 2 holidays a year.
Good luck!