Why treaty-era capital-ship trades required much more than Admiralty approval

How could the Admiralty trade the Iron Dukes and Tiger for more Nelson- or treaty-era capital ships?

It would need agreement across government, Parliament, the other services, and the treaty powers, and the old ships would not exchange evenly for new construction.

Treaty-era fleet substitution was a political bargain, not a simple ship-for-ship exchange. Treasury, the other services, Parliament, and foreign treaty partners would all have to accept the proposal, while three older hulls might purchase only two new ones in practical tonnage terms.