The answer to your question really depends on what type of food you like, what kind of atmosphere you want, and how much money you're willing / able to spend.
Victoria and Albert's is a 5-star restaurant with incredible food and service - it's a wonderful place to celebrate but it's expensive.
California Grill, Jiko, and Flying Fish are signature restuarants with great food, depending on your tastes, and they are also great places to celebrate and more affordable than V&A.
If you want something informal but still good I'd recommend Rose and Crown or Raglan Road.