So A is the manager. The manager sent a mail to Employee A about concerns with the Agency - which is more than likely something that the agency would NOT like to hear.
In the meantime the manager sacks A , so A get on his high horse and sends the mail onto the agency to get up the managers nose.
I dont think this falls under the data protection act. It depends on the information that was sent. if this information was about performance of the overall agency, then I dont think their is anything you can do. If the information was sent outside of the company - example to another company with no affiliation then maybe something can be done but it would depend on the content.
I would just let it go and take a lesson from it. Never send a mail that you dont want someone else to read - if it is really that private.
If it is that private - their should be a way to block the mail from being forwarded or replied too. Otherwise , you could specify as a footer in all your mails - that under no circumstances should this mail be sent onto another party without prior consent from the originator. Either way I dont think this would stand up in court.
Data protection act is about protecting peoples data -like maybe phone number, address etc. I dont think performance falls under this category.
Hope this helps !