The Wolseley is very nice but a little old-schooly. If that's your thing, great. If you want something funkier, try checking out...
Pied a Terre on Charlotte Street... very expensive, very elegant, outstanding food.
L'Atelier Joel Robuchon near Covent Garden... quite stunningly expensive, but (seriously) a real gastronomic experience.
Nobu is still very nice: just off Park Lane, great staff, make you feel very welcome. Just make sure he likes Japanese food.
I've been to all three - Pied a Terre several times - and they're all wonderful. If you're pushing the bost out for a birthday that's great, but you probably won't make it out of any of these for less than £200 for the pair of you.
Just to append something to CountessDracula's post, I was at the Waterside Inn recently. The setting was lovely. The food much less so. Sorry, but I was rather disappointed.
Have a great time wherever you go. Enjoy.