>>> I thought all union members voted Labour
There are far more registered trade unions that are NOT in the TUC (which, of course, is affiliated to Labour) than there are within the TUC. Many of those registered trade unions will have a rule barring that union from any party political links at all.
While TUC-affiliated unions get most of media coverage (partly, of course, because they're often the larger unions anyway) there are lots of unions which will happily campaign on issues which are important to their membership (irrespective of which political parties may, or may not, support their aims) but deliberately avoid party politics, per se.