Erm, just because your house is worth £1milion doesn't mean you paid £1million. We're in the south east, outside of london in a 3bed semi, in the 5 years we've been here our house is now worth £100 - 150k more than we paid. We now couldn't afford our house. I can easily see you could buy a house in London 15-20 years ago that is now worth a fortune, but that doesn't mean you have a fortune in the bank, you own an asset that is on paper worth that, but only if you sell it. And then you'd have a lot of money, but nowhere to live.
And if you have to sell to pay the tax, you'll have to move out of London if you need a similar sized property.
London/SE property is unusual in this way - in another 20 years, then yes, its unlikely you'll have anyone living in those expensive houses who didn't pay a lot of money, because the older people will have sold up for retirement/died off.
But now, you do have people who were just 'lucky' to buy when they did, I don't think a policy that is going to drive out the middle classes from London/nicer areas of the SE is fair. Eventually, (unless something significant changes) we will have a London that's reserved for the very rich or the very poor being subsidised, or the few middle classes holding on in very small properties compared to what colleagues in the rest of the country live in, just so they can say they live in London, or younger middle class people in rented house shares, before moving out when they have DCs - but I don't think this is a process we should be aiming to speed up.