I don't pretend to know how it works in the UK.
Here in the US, disassociation is not an exercise of ecclesiastical authority.
It is a voluntary action initiated by the congregant and the function of the Elders is only to confirm that the individual really meant it.
Unless things have changed (?) I'm not sure how the letter you guys are taking about would not be construed as a formal disassociation in and of itself.