The Japan Centre, on London's Regent Street is truly an Aladdin's Cave. Packed to the rafters with all sorts of Japanese food and drink goodies - you'd be hard pressed to find anything they didn't stock. With my new found love of Japanese cooking, I'm pretty sure I'll be back to fill up my cupboards.
Now this isn't exactly an undiscovered gem, I've known about it for some time but just haven't quite got round to visiting - largely because I'm rarely in Central London these days (due to living in the sticks and working in the ar*e end of nowhere).
You'd don't have to be into cooking your own Japanese food to make the most of Japan Centre. At the front of the shop is a vast array of sushi, sashimi, tempura, gyoza etc etc, for takeaway or (if you're lucky, or it's not lunchtime) to eat at one of the wooden benches. The range is impressive and it's much cheaper than Itsu or Yo Sushi!
Whether you live in London, or are just visiting I highly recommend a visit to Japan Centre. I just wish my office was closer!