But I really can’t imagine that it is lawful in the UK for an employer to have on his office-wall a graph that shews the month by month decline of numbers of Union members.
There's no breach of information governance or the Data Protection Act if an employer chose to have that graph as there's no personal information.
Any employer who has any form of collective arrangement with a union has to know how many are in the union to ensure that the union being negotiated with is still legitimately representing staff.
It's up to the union to ensure that this info is up to date.