My mum has Parkinsons.
Parkinsons is not a diagnosis of exclusion (will explain more below)
And Parkinson's is not diagnosed purely on symptoms (ie what a person feels and experiences). It is diagnosed on symptoms and signs (signs = objectively observable).
Please do not take this to mean I am dismissive of hidden/invisible illness, because I'm not. I understand the challenges in getting people to take ME seriously, and it is horrendous. But I am frustrated that you are using Parkinson's to argue your point about ME, without understanding of what Parkinson's is.
Parkinson's is not a diagnosis of exclusion. It is diagnosed clinically. A consultant neurologist carries out a physical / clinical exam of the patient. The signs and symptoms are consistently observable within the clinical setting, eg intention tremor, rigidity, handwriting size, and in more advanced cases, a characteristic masked facial expression, quiet voice
These are symptoms shared by a group of disorders known as Parkinsonisms. Parkinson's Disease is the most common type of Parkinsonism. Other forms include (but are not limited to) drug induced Parkinsonism (especially amphetamines), also strokes and Lewy Body Dementia.
The neurologist is usually able to determine with good certainty which Parkinsonism the person is experiencing. This is because of differences in onset, presentation, and medical history. Scans may be done if there are concerns that the cause of the parkinsonism is eg dementia, but in most cases they are not needed. Where scans are done, they are used alongside clinical examination and medical history. Medication response is a key factor. That is what the NHS info means when it states "there is no one test". There are a number of tests, scans and examinations which can be used concurrently and collectively to inform diagnosis.
So in terms of Parkinson's diagnostic procedures being comparable to diagnosing ME. It's like comparing apples and oranges.
With Parkinson's Disease: Patient sees GP about symptoms. Characteristic signs (eg tremor, rigidity) are objectively visible to GP. Consequently, GP refers directly to neurology. Neurologist carries out clinical exam and possibly (but not always) imaging. There may sometimes be a period of watchful waiting, but usually this is not the case. The Neurologist makes diagnosis regarding the type of Parkinsonism (Parkinson's Disease being the most common).
This very different to ME, wherein a patient might see a GP about often invisible symptoms (I use the word invisible to mean hidden, not in a pejorative sense), and the GP starts lengthy process of blood tests to rule out anemia, infections, malignancies, etc, then after multiple appointments refers to multiple / different disciplines such as immunologist, neurologists, rheumatologists, each if whom has their own diagnostic processes, over a timeframe of often many years.
Parkinson's is simply not a diagnosis of exclusion in the same way, and it is not diagnosed purely on symptoms.
As to questions on this thread as to whether Parkinson's is categorised as physical or mental health. It is classified as neurological, and, where psychiatric symptoms are present, it is neuropsychiatric.
I agree with you that ME should be viewed as seriously as other debilitating conditions. However (and I am sure you did not intend this) i find your comments around Parkinson's problematic, because whilst you are (rightly) frustrated and angry that others are ignorant about ME, you yourself do not appear well-informed about Parkinson's, but use it to further your discussion of ME.
If anyone is interested in learning more about Parkinson's Disease, it is well worth visiting the Michael J Fox Foundation, or Parkinsons UK