I don't know if it would be too far off your itenary Lutz but my favourite site in Scotland has to be Carradale Bay C.C. site on the Mull of Kintyre peninsula. The drive is scenic though, mostly skirting the lochs and sea.
It's not regimented, and the pitches are on grass which is kept cropped by the semi-tame rabbits. I must admit that my choice could be biased by the 9 days of wall to wall sunshine we had and there were Rhododendrons everywhere.
The beach is about one mile long and sandy, also sheltered. The seawater was tepid when we were there last June.
We also spent a pleasant day on Arran, catching the ferry from Cloanaig.
North Ledaig is a nice place to stop, although the pitches are regimented. We went to Mull for the day from Oban while there and that was nice driving around the island, also having a meal at Tobermory.
To add to Lady Mo's post about Onich, the beach is made up of big stones and if you want a lochside pitch you cannot erect an awning.
Two other sites I rate highly in Scotland are the C.C. site at Dachalm, Brora and the C.&C.C. site at Rosemarkie, near Inverness. Both sites have long sandy beaches with hardly anyone on them. To reach the beach at Brora theres a gate by the pic link I gave below which takes you 2 minutes walk onto the beach. Brora has some quite scenic drives around Loch Brora and Glen Loth. We drove up to John O Groats from Brora but it wasn't worth the bother :O(
A big plus at Brora and Rosemarkie is the absence of Caledonius Midgium.
My favourite site that I've visited in the lakes is Low Manesty. People are put off by the single track double humped bridge at Grange but it's no problem. From Grange, once over the bridge, the road is narrow but it's not far from the site then. Turning right out of the site to go to Keswick there are some lovely views. You can walk directly from the site down to Derwent Water and into Keswick. The downside is theres no shower block if you must have one.
See the following links Lutz, hope it's of some help....
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=14
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=20
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=0
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=1
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=2
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=3
http://www.carryoncamping.org/gallery/displayimage.php?album=89&pos=4
http://www.carryoncamping.org/gallery/displayimage.php?album=87&pos=0
I hope the links work and that you don't need to be a member to view them. Double click to enlarge.