The People's Palace in the RFH was really good - a good few years ago ... I assume it's still there - fantastic river views too - and they probably offer pre theatre bookings
There's also a Yo Sushi up by the London Eye in the GLC complex
Or, dare I suggest leaving SE1, and go to Gordons wine bar on Villiers St by Embankment tube, for a slap up meal of cheese and stuff and fantastic wine and then wander over the footbridge ... very sparkly and Christmassy views!
Yo sushi is quite possibly the worst place on the planet to eat sushi... I'm willing to argue this with anyone... but then again, if anyone knows of places worse then Yo Sushi, then I don't need to hear about them either...
I agree that Gordons wine bar used to be a very nice place though !
The People's Palace is definitely still there- I took my girlfriend there earlier this year for a champagne lunch. The food was good (and decent portions - not just small portions stacked high a'la 'trendy food') and the service was excellent. The RFH has the feeling of being in a school building, and this permeates into the People's Palace, but I think I noticed that more because it was lunchtime and it was empty.
If they can maintain the quality of food and service when it's the evening, then I would imagine it would have a lovely atmosphere about it at night.
The Peoples Palace is lovely at night, and quite romantic. It is especially worth going in winter as your view of the river is not blocked by the trees. They can not reserve you a window seat, but I find if you mention it when booking you will get one.