I like the following for their Japanese desserts:
Abeno for their Anmistu
Sushi of Shiori for their home made ice cream. Their flavours include black sesame (nice and creamy) and chestnut.
Necco on Exmouth market for their homemade desserts. I love their green tea tiramisu and green tea chiffon cake.
There used to be a decent takoyaki stand in russel square on saturdays but I haven't seen it in a while.
Jade Garden isn't bad for dimsum but it's not a place to linger. There have been several times where my dining companions & we asked to clear the table as soon as we paid our bill so that they could seat people in the queue.
I haven't been to too many other places in Chinatown for dim sum but if you don't mind schlepping down to South of the river and braving the Elephant & Castle roundabout, I highly recommend Dragon Castle. It's as close to Hong Kong Dim Sum, by way of Vancouver, that I've found in London.