Hmm seems to me you want to try and develop new blends of tea to bridge the gap between British and Japanese teas. I think that is a good idea you can get some pretty interesting tastes like Earl or Lady Grey and Lapsang souchong as examples. That or maybe run a taste test of various tea types and try and see what would compliments each thing on the afternoon tea menu they come up with a recommendation list.
To add to this its a good idea to try and complementary side dishes however on top of the rice cake idea look at introducing some Japanese Style sweets like Mochi, Manjū, Dango and Daifuku as examples and then see if they can be tailored to British tastes in a fusion heck could even spawn a luxury spin of business with a novel take.
As to the usual Jam, cream and scone used in afternoon tea see what can be taken from Japanese tastes to be used in a fusion like instead of strawberry or Rasberry jam for instance maybe try something like adzuki bean paste instead since it is sweet and is used a sweet filling in many confections.
Hope this helps