I've met plenty of idiots with qualifications and it was never an experienced man that ever built anything new or brilliant. Getting into a cock measuring excercise about electrical qualifications is the road to ruin. PV is not that complex you do need to do a lot of reading or learn from someone else. The courses are just an introduction but to say that someone who's got level 3 is by some measure capable of solar PV without specific training and/or experience is hopeful to say the least.
PV is just another special location within a book of regulations and as such I would think that a competantly trained electrician would be able to install a system pretty easily. The same as a Marina is or an Agi installation, I can count on 2 hands the number of Agi installations I have worked on in my nearly 40 yrs in the industry but the ones I have done were safe and fit for use, by working to the regulations, and relying on my training.
The design of a PV installation is perhaps a little more involved and need for extra and specialised training IMO is needed. Again though I know having done the EcoSkies course that there isn't a great deal more to it than a few calculations, which again most competant electricians would grasp easily, and being able to read and understand technical manuals. The MCS acreditation is perhaps a little more intensive than joining a normal Part P scheme, but again as the MCS survivie on the installers money to keep in business your assessed to pass not fail.
What is though the defining factor is, non electricians that do the PV course, will struggle when it comes to knowing what is not in section 712 or what is not on a course. Once you start working on Mrs Jones electrical installation the game changes. You need to know about bonding, you need to know how to complete testing on the AC system, what needs RCD protection and what does not, how to complete an EIC and a schedule of inspection and results, all things competant electricians do.
It is unfortunately like everything else in my industry, you now have Fire alarm engineers, UPS engineers, alarm engineers, Domestic Installers even emergency light engineers, and now PV installers, where all you really need is a competant, trained electrician, and not a semi trained "engineer"