I like: Mitsukoshi

Step out of the hustle and bustle of London’s Lower Regent Street into Mitsukoshi, is like stepping into a little enclave of Japanese ordered peace and calm. The store is part quality souvenir store where Japanese tourists can pick up Cath Kidston, Mulberry and Burberry  in more pleasant surroundings.
Mitsukoshi since 1673!
There is also an ANA airline travel desk, Japanese tour organisers and a Japanese book and gift shop on the lower ground floor which is why I like it so much.

Once all the retail therapy is out of the way it has one of London’s best restaurants in the basement. Just visiting Mitsukoshi is a cathartic experience, conversely even the most gentrified aspects of London seem barbarous in comparison.

Mitsukoshi (U.K.) Ltd
14-20 Lower Regent Street, London SW1Y 4PH

More information at their UK website; the restaurant has its own website here.