Hubert... how are your plans going?
If you want to get off the tarmac, options in UK are fairly limited... restricted to Byways/greenlanes, and private estates or 'pay and play sites'... there are some established companies that offer Tag along trips - Yorkshire, Lake District, Wales and Salisbury Plain (Stonehenge) are the regular areas they run through.
I have been keen to explore Scotland, but much of the rural areas seem to be private estates and forestry... I have also heard that wild camping is no longer allowed?!...Bothy's are available free, and I think there is a website to locate them...but not many will be accessible by car, mostly for walkers.
The upside is that the single lane tarmac roads are widespread, and offer good access to most areas, and will not damage your vehicles - so a good chance of getting home... but you will be sharing them with multitudes of normal cars, especially in the summer.... avoid school holiday periods if you can.
I went to Devils Bridge, near Aberystwyth in Wales between Christmas and New New Year... I booked a day out with Cambrian Safaris, and Richard was a great guide, showing us around the area, describing the history and geology...the Yswyth and Elan valleys were as stunning as I have ever seen, and no other people - even in the peak summer!... so much better than previous '4x4 guides' that seemed more interested in covering as many miles as possible off road with no local information.
If you run out of time to reach the Highlands of Scotland, at least visit Arran... Scotland in miniature.
Happy travels... and don't forget to post some pictures on here when you get home.