Is there a third party program, that allows to view all the available Skins for that type of aircraft prior to selection

Not so much for the British aircraft as aside from squadron markings, there are 32 shades of Brown

But the German aircraft can make colorful targets ( especially in QC ) as there you control which type of aircraft you are going to do battle with

Because right now, much of that 15GB of files goes to waste

Yes, there is.
I use IRFANVIEW for that - it's very easy to use, and it's a free download.
Here is the link:

http://www.irfanview.de/

You will find the skins here:

OBD Software > CFSWW1 Over Flanders Fields > campaigns > CampaignData > skins

A handy feature but may barf your PC is once you are browsing the folder, press T for thumbnail then wait for 485 hours whilst it reads all the 4000+ files. So might be better, if say you wanted to view Albatros skins, then copy just those to a temporary folder and view from there, then pressing T will be quicker.