Edit: yeah, OK, you can't buy a mansion in zone 1 for £1m. I'm going on my own internal standard, i.e. if it's over £1m it better have turrets.
Don't forget all these charts provide averages across all central London areas - some more real-life examples:
A terraced house in Earls Court cost you about £150-£200k early 1990 - now £1.5M+ - similar in Fulham etc.
A double fronted house in Worlds End (now considered to be part of Chelsea) was about £350-£500k at the same time - now about £4-6M.
Many people working in central London until about 20years ago bought themselves property from bonuses - these properties (first time buyers) were mostly used for self-occupancy - now there are very little bonus payments for younger people working in the City and a large proportion of people working in offices in London commute between 2-4h each day (when the 9-5 becomes 5-9).
Now (since at least 6 years) most property is cash-bought - i.e. no mortgage was taken out. You are looking mostly at "investment buyers" and with these, prices are far more sensitive to changes like the Brexit. Look around in central London and you will realise how much residential property is actually empty - for some of these "investment buyers" it is cheaper (tax) not to rent and the government is not doing anything about this since many years.
Most property development - to my knowledge - is now "to-rent" vs. "to-buy" - what we are seeing is a reversal of policies supporting people (latest Thatcher) to become property owners back to pre-WW2 when only the rich could afford not to rent.
On current central London price level you will have to have best at least £500k cash for a down-payment and then earn at least £200k p.a. to be able to pay your mortgage for a family size property.
In terms of empty property, this is what the Land Value Tax is good for (especially in C London, were most of the house price is for the land). Its a shame the Lib Dems never win general elections.
But back in 1990 interest rates were a whopping 14% - that means that a 25 year mortage would have cost the equivelent of £4935 each month.
Today that house is worth 1.5 million. The same 25 year mortgage at today's interest rates would be £5158 a month.
So really house prices haven't changed that much, it's just that the money is going to different places.
1 Million will buy you a 3 bedroom flat in Zone 1.