We have been to the Portrait Restaurant, in the National Portrait Gallery quite a few times now (for breakfast, brunch and lunch). It is on the top floor of the Gallery on the left hand side, going up the main escalator and taking the lift to the 3rd floor will get you there.
The place is normally busy and buzzing, so best to book. It also has a good view of Nelson’s Column.
We went for afternoon tea, taking up a special January deal of £12.50 per person for a Champagne Tea. A Bargain!