Hardly anybody ever posts about chocolate here except when travelling to Belgium or Switzerland. Shockingly to anyone who think British chocolate is only Cadburys and Rowntree, there are now quite a few places in London to buy chocolates of just as good quality as what you'll get in mainland Europe.
First of all I need to mention Alexeeva and Jones (297 Westbourne Grove in Notting Hill, near junction of Westbourne Grove and Portobello Road). This is a chocolate shop that doesn't produce its own, but has a wide range of superb and unusual chocolates from the UK, Europe and the USA.
There are also four major British producers with boutiques in London:
L'Artisan du Chocolat (branches at 81 Westbourne Grove, Notting Hill; 89 Lower Sloane Street, Chelsea, also concession at Selfridges and often a stall at Borough Market): very wide range of unusual bars, filled chocolates as well.
Paul A Young (branches at 33 Camden Passage, Islington; 143 Wardour Street, Soho; 20 The Royal Exchange, City): wide range of chocolate bars and filled chocolates in the UK tradition, some very unusual. Also high-chocolate brownies that are closer to French moelleux au chocolate.
Rococo (branches at 3 Moxon Street, Marylebone; 5 Motcomb Street, Belgravia; 321 Kings Road, Chelsea): filled chocolates in the UK tradition with some Italian influences, also a wide range of chocolate novelties.
William Curley (branches at 198 Ebury Street, Belgravia; 10 Paved Court, Richmond, concession in Harrods Food Hall): Chocolate bars mainly based around nuts and caramel, filled ganaches in French style. Also wide range of patisserie.
Foreign firms with boutiques in London:
Maison du Chocolat (46 Piccadilly; concessions in Harrods and Selfridges): French firm with French style filled ganaches and chocolate-covered fruits.
Pierre Herme (13 Lowndes Street, Belgravia) EDIT (now also in Covent Garden at 38 Monmouth Street): another top quality French firm. Also concession in Selfridges that mostly concentrates on their macarons rather than chocolates.
Neuhaus (11 Long Acre, Covent Garden; Exchange Shopping Centre, Putney; concession in Harrods): Slightly less upmarket than most here, but my favourite Belgian firm, personally.
Also Liberty's, Selfridges and Harrods have very interesting chocolate departments. The basement food hall of the John Lewis store in Oxford Street is worth visiting purely because afaik it's the only place in London that you can choose from a small selection of the way-out range of chocolate bars from the eccentric Austrian firm Zotter, who are probably the closest real-world brand you'll get to Roald Dahl's Wonka.