How did we do it? We spent less than we earned each month & used the balance to pay down the mortgage principal so we weren't spending our lives feathering the nests of bankers paying interest, while having nowhere to live in old age 🤷🏼♀️ There are convenient products available to spread the maths across, say, a 25 year period-called "repayment mortgages".
Being less flippant though, if the London market takes say a 20% dive over the next 10 years, keep in mind that at present all of that value will come out of your current 'equity' and you'll still owe the bank the same amount back. Would you be able to buy anywhere you'd want to live if that happened?
You've obviously become entirely too used to having more cash sloshing around than you actually have in your 'real world' family budget - particularly once nursery fees dropped away. Even if you'd put just half of one child's nursery fees towards overpaying the mortgage each month, you'd be most of the way through it by now.
Or put another way - one holiday a year & two bottles of wine plus 3 coffees a week.
I get that in London it must be very easy to be drawn into keeping up with the Joneses & having the children in the right activities, holidaying in the right places, eating out at nice venues etc etc etc.
But the kind of sums you seem to be talking about would have been absolutely transformed by knocking £600 a month off the principal for the last 8-15 years.
You've lost a lot of time, but you can still get there relatively quickly if you address your household budget and find a grand a month to stash (best done now before uni fees loom!) Make sure bonuses all go straight into paying down the principal too.
As your husband is self-employed, it may be worth looking at an offset mortgage-any money he saves monthly towards his twice a year tax payments can be sat offsetting the total interest you pay and then drawn down on tax day. Make sure this is OVER a general 15/20 year repayment plan though, rather than borrowing from the mortgage to pay them.
It sounds like a fundamental household budget rethink will be required, but you've been living on borrowed time, so you might as well crack on with it now - better late than never!