Definitely Japan in my opinion. They have one of the most distinct and most prominent cultures in the world. The UK is like a cluster of different cultures.
United Kingdom. Far more prevalent culturally in the world, whether we speak for the past or for the actual time that we live in. Music, food, fashion, language, books, history all speak in its favour.Japan is culturally rich, but its impact remains limited in the wide panorama.
I mean, Japan is huge and very influential. But England LITERALLY conquered the world and half the world speaks english.. Japan loves blonde hair, blue eyes and such too. They take great influence from American Rock and pop ( alot of which coming from Blues and Soul but that's another topic..). They were slayed by The Beatles too.