John Lewis....its a chain, so you may have some success. The one on Oxford St near Oxford and Regent comes to mind.
Micro Anvika at Tottenham Ct Rd and Oxford. Then there's that store that starts with an "S" (braindead moment....forgot the name) at New Oxford Street (or say Gower St) and Oxford, right near the Micro Anvika and TCR station. Harrods also has a selection. It depends on where you live, but those sound like a good start if you don't mind going central.