I love Benares, but I don't find it particularly romantic. I think it's the lack of windows.
If I wanted a romantic London restaurant a good view or outside space would be a big factor.
I'd look at restaurants such as Babylon at the Roof Gardens, Kensington; the Oxo Tower restaurant; Kaspers Seafood Bar at The Savoy with cocktails before in the American Bar; Orrery, Marleybone High Street or Coq d'Argent at One Poultry.
If you'd like any more suggestions I can keep thinking.
Are you going to share why it needs to be romantic?