Every 10 years the value of the assets in the Trust is measured and tax is paid on that valuation. Whether the assets increase or decrease isn't a disadvantage to the Exchequer as the tax is payable on the revalued assets. So the next 10 yearly charge will be a smaller % that then 40% due but will be payable on a larger value than 9bn (presuming along with you that Mayfair prices don't fall!

).
This is the bit that's confusing me about your statement, I can't make out whether you're saying the small % is charged on the original asset values (i.e. the 1bn).
it is charged based on the value of the assets at the time... which in this case, for the first payment, would have been circa 1 billion and for the second would be a few..
whereas if the estate was not in a trust there would have been a much, much larger 40% charge on 9 billion
point being so long as the assets keep growing like that over time then the 10 year charge involves much less tax
(obvs there are business properties and farmland involved too but that relief applies to both the IHT or the 10 year charge)
I mean lets say there was a 10 year charge when it was worth 1 billion @6% = 60 million... then 10 years later when it is worth say 4 billion = 240 million, then when worth say 7.5 billion = 450 million so total = 750 million
vs 40% of 9 billion = 3.6 billion
even then my argument isn't purely financial but also ideological - for example - the estate isn't going to have 40% in cash reserves, if faced with an IHT bill it would likely have to be partially broken up... which serves a useful function in itself as property comes back to the market instead of the most expensive parts of London being locked away (in theory forever) and not accessible.
Taken to extremes what if more people did this? What would society be like if a select few trusts owned all freehold property in London over multiple generations? Of how about all of the UK? The 'heirs' after multiple generations having no more dna from the original founders of the estates as anyone else but because of the will of some random person (or people) who lived hundreds of years ago say a large chunk of property in a capital city is secured, unavailable for purchase but just locked away in trusts for the benefit of some random line of male heirs. That is more than just leaving something for the kids, grand kids... it is potentially fundamentally affecting society multiple generations down the line.
We've gradually evolved from a feudal society, then moved away from aristocrats owning everything... I don't think trusts like this will necessarily survive many more generations as they're pretty regressive. It isn't really right that someone who is long dead can dictate some terms for the use of a bunch of assets in the capital city in perpetuity at least not when they're not being used for charitable purposes.
What case are you referring to?
https://en.wikipedia.org/wiki/McPhail_v_Doulton
didn't require a list of beneficiaries ergo discretionary trusts became more useful for estate planning... as they could be set up for the benefit of multiple generations of unnamed heirs
Although I do disagree with viewing trusts as the individuals property. Trusts don't escape taxation, they are just taxed differently which is the main point I'm trying to make.
I'm aware they're taxed differently and that is part of the problem I'm highlighting. I've not claimed they are individual's property I'm claiming that they're vehicles used to avoid inheritance tax and preserve dynastic wealth over several generations of heirs and that when used for this purpose there is not much benefit for the rest of society, in fact I'd say that preserving a massive chunk of dynastic wealth over multiple generations is rather a negative thing and I think further clampdowns are required.