Kei cars are never going to pass crash safety regulations outside Asia.
For true JDM cars, They can, but it often involves heavy re-tooling, and I only know of two-seaters/roadsters making it over, like the Suzuki Cappuccino, which was approved officially in the UK.
Otherwise, they're either whitelabelled trash (Peugeot iOn, Citroën C-Zero), different models (The Wagon R is not the same as the R+), or they're cars that
just so happen to barely fit kei requirements with minor tweaks (Smart ForTwo is one).
In the end, as cool as Kei cars are, they're just not what the markets want in droves, the US buys SUVs and Trucks, the EU spends on SUVs, Sedans, and small cars, and most of Africa either buys whatever is imported, or prioritize cheap, large vehicles (think Honda Element or Ford Transit Connect)
Europe may look like it's a potential market, but people don't buy "small" cars because they're small, they only buy them because they're cheap.
You can get a car with most modern bells and whistles for 15k GBP, or 130 GBP per month on lease, with the average yuropoor salary, it's the only option for many people.
Getting a kei-sized car up to standard would cost more than just adding more crumple zone.