Osteria Basilico on Kensington Park Rd - cosy Italian, fabulous authentic food, definitely need to book at weekends. Good area for a drink before/afterwards around Notting Hill/Portobello.
Gilgamesh in Camden is pricey but food's nice and cocktails are great.
Another pricey one, but well worth it, is Hakkasan.
The Blue Elephant in Fulham is a great Thai restaurant in Fulham, very OTT decor, gorgeous food. Again, pricey.
Odette's in Primrose Hill - modern European food, not cheap, nice for a summer's eve as close to the park and opposite a great pub. Limonia is a lovely Greek restaurant directly opposite.
Zuma in Knightsbridge is a glam Japanese, great for a special occasion