All of these. If you look at a database like the megalithic portal, or even our own /r/megalithporn, you see that theres plenty of similar stone monuments all over the world. However, the amount of megaliths is still a peculiarity of Britain, though I do not know what happens if you broaden the definition to also include burial mounds (Denmark might beat it) or runestones (Sweden might beat it).

Still, rather than just comparing and counting stone monuments over vast areas of the world, it would be more useful to look at the individual monument traditions in their own local region. The megalithic traditions (British Henges, Funnel Beaker megaliths, Atlantic menhirs and dolmens) do have their own regional inspirations and divergences, nevermind the chronological differences between them (we're talking about millennia-scale time differences between the later phases of Stonehenge and the first long barrows in Brittany, for example). So if you're looking mostly at henges, then yes, Britain has most of them because most of them were built in Britain. In this sense, your question is quite similar to asking why pyramids occur in Egypt.

Of course, environmental factors like the availability of stone is also important, as is the fact that many British monuments were built on heathland relatively immune to intensive agricultural practices or later construction activities.