Of course he doesn’t want you talking to a solicitor! Ignore that.
Based on what you’ve said, and all the reading on divorce I’ve ever done, I see no reason why you wouldn’t get 50% of the house and the pension. Let’s forget any other assets and child maintenance, or if spousal maintenance might be possible.
He must be shitting his pants!
Whatever your settlement, the date you choose to retire is YOUR DAMN BUSINESS. You look at the money YOU have, and choose what YOU can live on. The more frugal you are, the sooner you can retire - you don’t need as much.
So - a full paid off house in the SE that sounds like you can buy outright with your half? Nice.
The £1m pension. Even if it’s not more than 50%, that £500K get transferred to a different pension account in your name with a PSO. Nothing to do with him anymore.
You can access it at 55. Are you familiar with drawdown? Some people say the safe rate is 4%. That means, you can take 4% of the money every year and the growth in the funds will replace it. Not every year - it averages out. Others say that’s a bit high, but let’s take 3%. That’s £15K a year. (taxable) Alongside no mortgage, that’s not that bad. Not even allowing for the 25% tax free amount from a pension - I don’t want to go to into detail.
Fuck him. Go and see a solicitor. You’ve tried mediation. Check with your solicitor but you’ve probably done enough to satisfy the requirement that you try. No more little chats. Black & white, from your solicitor - tell what you want. And make sure it’s not a penny less than the highest amount your solicitor thinks is realistic.
And enjoy your retirement. It will be richer than his in many ways.