The asset split, assuming there is a decent pot, will reflect that they both need to get a home to live with their children. The cost of a home won’t change if the split is 60:40 or 50:50. So she absolutely won’t get a bigger share of the assets if they are with her more than him. This could change if there isn’t a lot of equity or if he doesn’t intend to see the children. The asset split is likely to be determined by their respective incomes.
Child support is meant to fund the cost of being the resident parent.
The purpose of the divorce will be to try to make the OP and her ex equal. That will roughly translate into giving them enough equity to afford similar property in same area (the school proximity will dictate that). How they choose to use that equity would be up to them. It sounds like he wants to stretch to keep an expensive home. The OP seems to want a more manageable mortgage.
OP, it is up to you both to decide how you want to split things up. If you both have informed advice and agree to a split the court will sign it off. Personally, I think you are being magnanimous in allowing him to keep the £75k given the property market you are in. But that is up to you. I also think he’s pushing it to keep the house and that is driving his need for the £75k plus c£100k. But this shouldn’t come at your expense. It’s a convenient house but is it the only option for him (or you).
At this stage I would suggest you advise him that you are minded to agree to ringing fencing the £75k but remind him that in court it wouldn’t be ring-fenced and you do so in expectation that he is as flexible. This is a big and significant compromise for you. He needs to be willing to do the same - be that over fees and moving costs or something else. I would also advise him that you need to assess what your housing costs are before you commit to anything including the £75k.
I don’t expect him to be happy if you do that. But remind him that what you are asking for will probably get you a property worth 1/2 million compared to his £1m. That doesn’t sound like equal.