I take it you're a vegetarian? As a veggie who enjoys fine dining I always have to do that extra bit of research as many fine dining / Michelin establishments have not taken to catering for vegetarians.
My favourites:
Jamavar, Mayfair - absolutely delicious unpretentious Indian food with a bit of a twist.
Hakkasan, Hanway Place - 'Stir-fry black pepper vegetarian chicken' is up there with one of the best dishes I have ever had. The selection of veggie dim sum and desserts are also exceptional.
Yauatcha, Soho - much the same experience as Hakkasan (no surprise as I believe they are owned by the same HKK group). 'vegetarian duck' and the dim sum were delicious.
Oxo Tower Restaurant - was astounded by the level of effort put in to their vegetarian menu, all courses were incredible, unsure whether they change it seasonally. I visited at the beginning of the year.
By Chloe - this is not somewhere to take clients / to impress. It's an NY establishment that's opened up two restaurants in London (Covent Garden / Unsure) doing vegan fast food. It is all very, very good. It's not based on the notion of fake meats like many other similar places but instead they do clever things with vegetables. Would highly recommend giving it a try.