Hi Jenny
Many advantages in being a member of a recognised workplace union or similar - eg

neu.org.uk/neu-membership-eligibility

Otherwise, such as Prospect may suit.

In reply to David:

What would you see as the advantages for HR having a union rep? For example, we are handling a grievance and there is an independent investigator who wants to talk to us all. (Grievance isn't against me). What would be the benefit of taking a union rep in with me rather than a colleague? I;ve never belonged to union before and have only dealt with unions when we've been dealing with disciplinaries etc.

In reply to Jenny:

I see the advantages as immense and the disadvantages few and usually not insurmountable. Unions have the resources if needs be to defend their members' interests in the highest courts in the land and often the contacts and employer relationships effectively to do so at individual workplace level. You don't usually have to use union reps as companions at meetings but they're typically far more experienced and better trained in that role and often HR folk representing themselves is a bit like doctors being the worst patients on earth!