Poule au Pot is the standard answer to the most romantic London restaurant, and I'd go along with it. However, anyone who half knows London restaurants knows that is what it's famous for. So it might be a rather obvious statement of intent. But then again, that might be exactly what you're after.

It's a nice place if you've not been before. Reassuringly Gallic. Pricey, but not ruinous.

OXO tower is only for the easily fooled. Deeply ordinary food, high prices.