I was invited to a meeting with my two line managers via email to discuss the customer service standard within my team. This related to a single incident two weeks ago (I only found out this was the subject at the meeting) the incident I had already addressed with the staff involved at the time. When I arrived a discovered that someone from HR was there to "take notes" I did not know this in advance and felt very uncomfortable as it seemed to formalise the meeting.
Can HR be invited to my meeting with boss without me being told in advance?
Yes. Unless there is some unusual local law stating otherwise, your permission is not required for HR to be present.
But if you are in a union, you should refuse to continue the meeting without your union rep. When in a union, always have a rep present for all "formal" meetings.