I'd pay off our mortgage, our siblings' mortgages and that of my best friend. That would probably cost about £750k. I'd buy a house outright for a colleague whose husband is disabled, have it adapted for his needs and give her a lifetime interest in it. That's about £500k. I'd give both sets of parents £1m in cash, so that's £3.25m to start.
We'd buy a house like this one:
www.rightmove.co.uk/property-for-sale/property-36414744.html
but live in our flat until it was redecorated, furnished and childproofed to my exacting standards. Call that another £3.5m all in, so I'm at £6.75m now.
Then I'd buy a flat in central London, perhaps like this one:
www.rightmove.co.uk/property-for-sale/property-34881789.html
and use it as a crash pad for late nights out and for friends visiting London. Call that another £1.5m with fees for all of the property etc, so still £11.75m left.
Then I'd buy a tiny seaside flat somewhere on the west coast of France, a big country house in Provence and hire someone in each place to manage them for the parts of the year that we're not there, pick us up from nearby airports etc. The French property and the central London flat would be available for close family/friends to borrow.
That might leave me with £10m. I'd put £5m aside in savings and with the rest, I'd fund a full-time member of staff for the animal charity we volunteer for, and give £1m to each of Save the Children, MSF, Shelter and the Trussell Trust.
I'd keep working, but step down from my current role and let them promote my lovely deputy. I'd go part-time but on a sort of consultancy basis - they call me in when they need me and pay me when they do. I'd love that. Between my salary, the interest on the savings, the lack of mortgage and the sale proceeds of our current flat, we could easily manage the upkeep of all of the property we'd now own.
I would be as happy as a clam.