The UK has found a new strain of Delta called AY 42, which is weaker than the one that is dominant around the world.
It is expected that within 8 weeks or more this new strain will become the dominant version of Delta in the UK.
It is apparently more infectious, but less powerful. More people are asymptomatic and don't even realize they have Covid. This means less self-isolation as well, so it will be harder to stop the spread.
Japan's drop in Covid cases may be caused by the virus mutating into a weaker form.