I should have some interesting stats soon for the U.K. thanks to the charity website and it's beta version. Many congregations have merged and been delisted as charities. Most of the consolidation has been going on since 2015.
The Beta version of the U.K. Charity website has the number of members/volunteers for many congregations as well as the number of trustees/elders so it's now possible to calculate a ratio of elders to publishers.
Truthseeker