That's a whole 'nother discussion; the point is simply that an ad hominem vs. the economist on this matter seem incongruous - and, more importantly, a distraction in poor form.

But I won't dispute that the idea seems applicable more broadly, especially since it's so much simpler and less fraud-sensitive than capital gains taxes. And while it's politically tricky, if implemented with a slow ramp up (i.e., don't do what Thatcher did) it probably won't hurt critical constituencies too bad. You could think of something like an X% tax of value above threshhold Y, and start with X low and Y high, and creep from there; e.g. X 1% and Y 500k pounds, conveniently hurting the rich not too much, and the bottom 50% of households even in london not at all. Even just the message that there's political will to do something about this would likely deflate the bubble a little.

The land property tax is intended to address specific issues with land value due to it's fixed availability and role in the economy. For example it's very hard to substitute unproductive land with something else, because you can't (to any significant degree) create new land, and certainly not with the same locational characteristics as existing land.

These aren't attributes shared by stocks or other financial assets. For example it's not straightforward to charge a rent on stocks, and they're not fungible to different economic uses in the same way land is.