Political Contributions
If I recall a part of trade union reform introduced by Margaret Thatcher was to prevent a percentage of union subs by default making a contribution to political parties at the diktat of the TU concerned. Legislation was introduced which meant if an individual wished to make such a contribution they had to opt-in instead of making a passive payment in many cases unaware they were doing so.
Now it could be that that piece of TU reform was amended by the current government. Individuals who feel strongly about this should of course contact their respective TU and find out the current situation.