Can PTSD be mild? Do the traumas have to be of a certain kind? As far as I know, PTSD was first used as a broader label for symptoms associated with so-called shell-shock, and has been diagnosed for symptoms of a milder nature.

But is there a lower limit to the point at which PTSD can be diagnosed? For example, is it possible to have such a mild form of PTSD that it does not interfere with normal functioning, but exhibits the same features?

The reason I ask, is I live in the UK where the National Health System (NHS) is increasingly struggling with funding. Their website on PTSD seems to only state the most extreme of traumatic events as possible causes. In short, I think it is to get out of giving help to all but the absolute worst cases. Hospitals and many mental health services have large burdens and turn many seeking help away.

If someone where to suggest they have PTSD, I feel that with literature such as the above, it belittles anyone experiencing "mild" events such as domestic abuse, which barely fits into their categories. Not only might they not get treated, but experience or perceive scorn.

If one were to seek help for PTSD and have to justify the help needed, regardless of the cause, how might it be done? Could the symptoms be misdiagnosed as, for example, regular stress, or anxiety, and would treatment for those be different to that required to help with PTSD?
In order for something to be diagnosed as a disorder by a mental health professional, the client typically experiences distress and impairment in activities of daily functioning. 

If the time frame is under 3 to 6 months, please consider "Acute Stress Disorder" as a possibility. I agree that the DSM V is going to be a useful resource for diagnostics.

Trauma, in any form, does pose very specific challenges for diagnostics and a treatment plan. For further reading on trauma therapy, I highly suggest "Life After Trauma: A Workbook for Healing" by Rosenbloom and Williams (2010). Treatment will be dependent on the practitioner's training, and the formulated diagnosis. You are welcome to research efficacies of therapeutic interventions.

Therapy is very useful for any and all of the items you have mentioned. If you are attempting to get insurance coverage, usually a diagnosis from a qualified professional will qualify you for therapy (at least here in the United States). A good diagnosis could take a few sessions to formulate. It's sometimes a longer process if someone suspects trauma.