Hi There aren’t many ‘white collar’ etc trade unions left, and much may depend on the particular sector in which you work. That said, Prospect Union might be appropriate: traditionally Prospect represented professionals in the public sector, such as scientists engineers meteorologists and air traffic controllers but they’ve diversified considerably since and might possibly suit

UNITE were always good for HR - not sure how they are now though as it's been a few years.

Which field are you in? I'm in education and as a Bursar then HR Manager I was eligible to join the Association of School & College Leaders.