I believe this review is relative to what you're accustomed to, but I've lived in the UK long enough to miss good sushi... and I'm from Georgia, not exactly the sushi capital of the US. That said, I have to say that I've been pretty much shocked at the "meh" factor of most sushi places I've tried in the UK... I'm sure there are hidden gems, but I've hit so many duds in my quest that I subconsciously have started avoiding one of my favourite food types.
So, on to Kanpai... it's good, really good... good enough that it's getting five stars because I simply haven't had sushi this good on this side of the pond yet, and it's reasonably priced. If you're over from Japan (or even California) I'm not sure it would rock you're world, but if like me you've grown cold to sushi because it's always either far overpriced, simply not good, or worse... both, then bookmark this place and give it a go.
The King Prawn Tempura... wow, it's a must for a starter, a real treat. I've had the Squid Tempura as well, and while also nice it didn't quite live up to the King Prawn Tempura... which is a bit of a surprise to me since I normally fall on the side of squid when playing favourites between the two. I've also had the Grilled Aubergine in sweet Miso Sauce and it packed a punch with flavour... it could have almost been a dessert! I've also had the Pan Fried Veg & Chicken Dumplings on two occasions, loving them the first time... sadly the second time, while still good, they didn't live up to the first visit... they came out a bit cold and the dipping sauce was a bit weaker in flavour. (They were still good, just not as good.)
(Side note... if you're in London, near Angel, seriously try the dumplings at New Culture Revolution... they're handmade bits of win, no other way around it.)
I've had a few of the rolls at Kanpai, but so far the Dragon Roll is probably the winner in presentation and taste... I don't think you could really go wrong with any of them that sound of interest to you. The Kanpai Special Combo is a win for sashimi taste and presentation... go with a friend and order one of these, a Dragon Roll, and have the King Prawn Tempura as a starter (I'll let you pick the second starter on your own... it's your dinner after all!) and your table will look like a presentation fit for a magazine.
So search no more Dear Yelpers, there is now somewhere I honestly look forward to going for sushi... not in the "well, it will do" way that so many other places have been until now... I legitimately crave this place.
For the place itself... it's almost hidden, with no sign jutting out, but clean and well laid out inside... the staff are courteous as well, but it can get quite busy... a booking on a busy night is wise... very wise indeed.
Enjoy!