By chance I have been researching Japanese cuisine for a client, that client writing a book of some sort involving a trip to the Japanese mountains above Tokyo, yo Okutama - actually very pretty up there.
Your are correct in your supposition, fgt. Tea ice cream and red bean paste being the closest they come. I attach a page of other options for you.
http://www.yummly.com/recipes/japanese-desserts
An alternative is to make a layered chocolate cake and then take care in decorating it - for example slice it and reassemble, each slice wrapped in some gold coloured paper and then adding some little decorations as well - gold leaf is perhaps going too far but in Japanese dept stores, you will find beautifully presented slices/cakes going for up to £30 per slice!
As to the green tea, does Waitrose have any green tea matcha, otherwise you could use a concentrate of green tea teabags, say with about four, tasting to see if it is strong enough.....I would say reduced to about three or four tablespoons worth to add yo your cream mix.