FYI I went solo a year ago and all I needed was to have passed the Air Law exam and have a medical.
There were some complications in getting the Class 2 medical - I had something in my history that needed more investigation - so I used the NPPL self-certification form in the meantime to go solo. Had no issues with that as they just hold you to the same standard as a professional driver.
The NPPL medical is a self declaration that you are medically fit. It happens to be the same standard as professional driving, but that doesn't mean you can just sign it and forget about it, if you know you have a condition you shouldn't be signing it, although your GP would offer advice it would be you who are liable.
Also, there is no legal requirement for air law pre solo, but it is often a club rule.