'why the main concentration of megalithic remains are on these islands - yes, I have the european book too'
I have seen megalithic remains* on five continents and can categorically state that the main concentration of megalithic remains is NOT on these islands**. Megalithic remains are widely scattered across the planet wherever humans have inhabited.
*Presumably you mean ancient or 'sacred' stone places?
**Presumably you mean England, Scotland, Wales, NI, and Eire?