For my Birthday we went for dinner at Bumpkin in Notting Hill.
A lovely country style restaurant that serves seasonal British food.
The atmosphere is perfect. Not too busy and not too empty. The lighting is cozy.
The food is also very good. I had the beef burger with bacon and it was superb.
The staff is friendly and attentive. The only mistake our waiter made when on my 30th birthday I claimed not to be able to finish my dinner was to tut and claim to be 40 years old while looking like Johnny Depp in his 20's.
If you do go I would suggest eating upstairs unless you're with more than the 2 of you.
Downstairs is more vibrant and lively whereas upstairs is more intimate and subdued.