There are hundreds of restaurants....Sheeky's is my favourite for seafood...a bit pricey and can be difficult to get in at short notice. Have a look at www.london-eating.co.uk they have a 'romantic' choice list
We had a lovely meal at [broken link removed] & they've a few loactions around London - we ate at the Belvedere Road restaurant which is 5 mins from the London Eye - we had a private booth at the back and it was gorgeous