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well it all depends on the food you want to enjoy.

if budget is not an issue try Nobu:
http://www.noburestaurants.com/london/experience/

Ramen
Wagamama is always ok, of course http://www.shoryuramen.com/ is very good too

okonomi yaki
go to my friend http://www.abeno.co.uk/

kaiseki
https://www.zagat.com/r/the-shiori-london

Soba
http://chisourestaurant.com/cha-soba

In europe, Paris and London are the best spots for japanese food !! Enjoy!

Excellent Thanks I will save these links for next time I go:)