There is so much that isn't clear about the past. So many things come to light as time goes on. Absolutely fascinating.

We consistently underestimate what ancient people could do and how far they could travel.

being in contact with each other.

Writing was independently developed in Egypt, China, and in North America before any of those people had contact.

The Babylonians, Asian Indians, and Mayans all developed zero as a placeholder independently.

The human mind's capabilities are the same the world over, so it's possible for people in different parts of the world, having no contact, will come up with similarities of thought and construct similar things for similar purposes.

But, if the British and Japanese stone monuments actually did have a common source (which I do not think they did), it could have come from location between the two distant lands. Somewhere near the Caspian or Black Seas, for example, with some people going east and some going west, carrying the concept with them. I doubt very much that it happened that way, though.

Just a case of "great minds thinking alike."

Yes it could be that humans are hard wired for certain cognitive and behavioral stages of development.
And will develop language, reading, tools, art at similar points in time.
Even with no outside contact.

Or it is possible humans were traveling and moving around the world earlier with more frequency that we
currently know.

Early Native Americans were traveling rapidly and efficiently all over the United States via the waterways.
It was like an interstate transportation system for them. If they hit shallow areas they could lift their
canoes and run great distances this way.

Current researchers and historians tend to be egocentric and minimize how advanced ancient peoples were.
And certainly, historically it is the case that white researchers weren't going to be bothered with POC history
and their accomplishments.

independent developments.

The indigenous people of the Americas did indeed travel all over the 2 American continents and maintained extensive, far reaching networks. That is how corn (maize) spread from its place of original domestication in Mesoamerica to people all over North and South America.

The independent development of the concept of zero in Mayan society is an example of how advanced some people were in various parts of the world. It's the people who discount the intelligence and capabilities of Indigenous Americans who insist that they could not have developed their civilization and mathematical skill in calculating the timing of a year, which was more accurate than any place else on the world. Those people claim that the Mayans must have got their knowledge from some vague, nebulous, imagined contact with civilizations elsewhere in the world.

The same thing happened regarding the city that we now call Cahokia. It was a huge Native American city in the North American Midwest. The people had built great earthenwork mounds and temples. But the city was abandoned before Europeans arrived. As American farmers moved westward and encountered those mounds and remnants of the once great city, they decided that no "Indian savages" could have achieved such developments, so they invented out of pure imagination, with no facts, stories about who the former inhabitants of the city had been. Some said that they were descendants of the "lost tribes of Israel (a doctrine in Mormonism). Some said that they were the descendants of Atlantis survivors. Regardless of whom they believed that the city's builders and inhabitants were, they agreed that the "wild, savage Indians" had destroyed that city. Therefore, the Americans settling the area from the east were justified in taking the land from its inhabitants.

Perhaps ancient groups were not as isolated as we thought.

Spreading goods, food, ideas which were improved and enhanced.

Perhaps there was more interconnection between ancient peoples than we imagined.

Some of the new DNA studies suggest that the Vikings traveled further and more extensively
and for a longer period than we realized. Same thing with the Asians and the Pacific mainlands and islands.

the inhabitants of Britain and Japan in the OP.

"Perhaps" is fun speculation. But without corroborating evidence, it is just speculation and free floating imagination, not science.

The Vikings even reached Canada, but only 500 years before the Columbus voyage to the Caribbean. By their own records, the Vikings were in Vinland very briefly and could not establish a foothold. They were driven out by the "Skraelings" (their term for the Native inhabitants). They did trade with them but it was a very limited trade farther north around Baffin Island.

The Polynesians did reach the coast of Peru according to recent DNA studies, but not in ancient times. I do not recall the dates offhand, but somewhere after 1000 CE. That is about 3000 years after the earliest civilization in Peru at Caral. And, although the Polynesians are the greatest navigators in the world, they did not have any aspects of civilization to bring with them to Peru, which was already advanced beyond the Polynesians. In fact, there is some evidence that Polynesian contact with Peru apparently spread maize to parts of maritime Asia, carried back home with them.

That Polynesian contact left evidence in DNA and in maize. The Viking contact left evidence in Newfoundland.

There is no evidence that any of the civilization developments in the Americas came from any place but America, independent of other parts of the world.

We see facts we cannot explain, so we speculate about possible reasons for what we are seeing.

Then we test it out with rigorous research.

The "perhaps" is not just for fun, it is the beginning of the scientific method.

Our most brilliant scientists have such incredible imaginations and thought processes they can
connect dots like most others cannot. And explain phenomenon others could not figure out.

Yes I know the time line of Vikings, etc but the point is that current DNA testing is overturning previous
theories that were considered fact.

as analytical. And hypotheses are speculation. Einstein thought out and visualized his ideas before doing the math to test them out. Usually a speculation is prompted by something that is observed.

So this is a very general discussion of what science is or is not, without much disagreement on the generalities. Is there some specific point to be made about DNA and the OP regarding the similarities between Stonehenge in the UK and stone monuments in Japan?

Seriously, though, interesting read.

and found that the oldest ones in the world are at Gobekli Tape in Turkey. They are 11,000 years old (9000 BCE). They precede iron tools, pottery, and agriculture. These circles were ceremonial sites. There is no indication that people lived at the sites.

https://www.smithsonianmag.com/history/gobekli-tepe-the-worlds-first-temple-83613665/

Stonehenge and the Japanese site of Oyu are both from around 2500 BCE. The location of Stonehenge was used as a ceremonial site as far back as 5000 BCE, but the stones were not erected until 2000+ years later. So, coming from the same time period, it is unlikely that either of the two cultures, thousands of miles apart, copied from the other.

But the idea of stone circle monuments might have spread from Turkey to other places. There are stone circles in several countries on the European mainland, so the idea might have spread from someplace like France to the British Isles, in England, Scotland, and Ireland.

How the idea for stone circles reached Japan is a mystery. Maybe they are not from outside Japan but were independently developed by the Jomon culture in Japan.