Washington treaty swaps and modern battleships

If Britain offered to dispose of older battleships under a Washington Naval Treaty scenario, why would that be difficult?

It could work for Britain, but it would push other powers to demand comparable swaps and would leave more modern capital ships in service.

Britain could make the swap, but every treaty power would likely demand a comparable concession. The United States and Japan could finish ships already under construction, while Britain might be the only power laying down a new design. More modern capital ships would then remain in service and alter later treaty negotiations. The British choice would also depend on numbers: a larger class could move away from Nelson toward a slower F3-style design with 16-inch guns.