According to information shared by Christopher Dring on social media (which is part of Gamesindustry), Tekken 8 had a strong launch in the in the UK, beating Street Fighter 6.
It has to do with the popularity of Playstation consoles in Europe and the fact that Tekken 3 was so synonymous with Playstation 1. They have a synergistic relationship. I'm guessing SF is more popular in places (East Asia, US) where arcades were a big thing. They were never a big thing in Europe in comparison. So Tekken, which was always more about home console releases, dominated here.
With that said, several entries for Tekken had lackluster sales and it was with Tekken 7 it was somehow revitalized.
During the break out of the PS1 days in Europe ,Tekken was one of the games always used to push the more "adult" side, especially when Tekken 3 dropped with it's grunge style and soundtrack being heavily influenced by break beat styles, drum & bass, etc.
Makes me miss the days of each region having it's own marketing style, Playstation EU was the best back then, big up Psygnosis!