Your company is not at fault here. They are actually responding in a reasonable manner. However, you have been given bad advice by your counselor, which caused you to trigger your company's response. If this is not what you wanted, then you shouldn't have done what you did.
she had forwarded it to our head office HR department; my question is do these things constitute a breach of confidentiality by my employer
Your HR manager forwarding something to the same company's HR department is not unusual, nor any violation. They are one department, regardless of whether they're split over several offices or not.
unknown to me my new line manager was also present & where my email was read out aloud verbatim without asking my permission
While discretion may be desirable in certain personal situations, I can't judge this since I don't know the exact content of the email.
Your line manager is one of the most relevant people to include here, as they are expected to be acutely aware of your work, performance, and any obstacles you face in the workplace. Your current predicament very much falls under their responsibility.
However this HR manager instructed me that I attend a meeting to discuss
Let's say they don't plan a meeting. HR received a written complaint by you, and they did not follow up.
Supposing something happens in the future, e.g. you sue the company for workplace bullying, the fact that they did not act on your (earlier) complaint of bullying will have a tremendous impact on the company's position in this law suit, and the HR employee themselves will also face repercussions from the company by opening them up to this liability.
On top of that, there's the human element. Even if not done to cover their legal ass, your company might be pro-active in hunting down workplace bullying in an attempt to nip it in the bud before it causes even more damage to employees.
Either of these are very good argument that justify and/or effectively force the company's hand in investigating this complaint.
nor did I want anything done
What was the purpose of the email then? The HR employee who read it wasn't involved in the situation, and is being asked to not do anything with this information. So why involve them at all?
It makes no sense for this email to be sent if not to urge HR to action. Therefore, it's reasonable for HR to take action because of this email.
was not allocating blame to anyone
The email was about my very personal feelings, about becoming depressed and my state of mind, this was all due to being bullied at work by my director and
The two bolded parts very much contradict one another.
You accused another employee of untoward behavior, furthermore explaining the mental stress it causes you and how this is continuing evidence to a diagnosis of work-related stress that dates back at least 4 months ago.
If you indicate that a particular person is causing a problem of this magnitude, then you are very much accusing them and laying blame on them for being (part of) the problem.
I stated I was venting (as advised by my counsellor)
There are two options here: you misunderstood your counselor, or your counselor did in fact tell you to vent to your employer. Did your counselor explicitly tell you to vent to your employer? Or did they tell you to vent in general, and you decided that your employer should be one of the recipients of this venting?
If your counselor explicitly told you to do this, fire your counselor. They didn't just give you bad advice, but advice that harms your career and increases/perpetuates your psychological struggles.
At best, your counselor is woefully naive. At worst, they are negligently causing harm to you by giving you the wrong advice. In either case, the counselor is not the right counselor for you.
If your counselor told you to vent, but you yourself decided to vent to your employer, then no one is to blame for your current predicament, other than yourself. Everyone in your story is behaving the way they're supposed to. You just happened to do something without knowing the consequences, based on a bad call.
If this is the case, the best you can do is communicate this to the company, explain how you regret sending the email, and explicitly state what your intentions and expectations were. Apologize for the inconvenience you caused, and ask if it is possible for the company to cease their investigation into this issue.
It's possible that it cannot be stopped, if the company has chosen for themselves to investigate a potentially troublesome element (i.e. your line director). After all, if they bully you, it's likely that they're bullying others as well.