The average house price in London is currently £629,286, but I could believe that those who want to live in a posh area might have to pay a million or more.
1 million gets you a 3 bed semi in a suburb - a nice suburb on the outskirts of SW London like Twickenham, but nothing particularly posh. As cakeorwine points out, given his age it’s a reflection of the increase in value of houses, not disposable wealth. He could sell it, but then he and his family would still need to live somewhere.
Unless the donkey sanctuary land has planning permission it won’t be particularly valuable. It’s likely that this is the reason he could buy it.
However, to be fair, I wouldn’t be surprised if he is more financially stable than Johnson.