Going to need a bit more information OP, London has so much choice and so many restaurants that could be considered 'romantic' that you're going to have to narrow it down for us including an idea of budget. Are you after fancy/ high end with the chance for both of you to get dressed up and have starched table cloths and waiters with cloches etc? Or is your vibe more a cool cocktail bar with elaborate sharing plates? Or a noisy / chaotic packed out place where you sit super close? Or a little terraced alleyway with a secret tiny restaurant? I'd consider all these romantic but depends a lot on your taste and stage in relationship (in our early days DH and I didn't have much money so it was a lovely treat to go to Chinatown and get noodles or a few dim sum baskets from a small place or to one of the tiny family run Greek/Italian places on Charlotte Street with red tablecloths and a smiling Grandmother in the corner and drink cheap red wine, I'd consider these very romantic but if I was to send you there now as a grown up couple it might not quite hit your spot.
For what it's worth my top recommendations would be:
For old-fashioned luxury (and definitely not looking out of place/country visitor if you want to go very dressy, which is not the case in a lot of London now), The Ritz
For art deco glamour, booths, champagne, Bob Bob Ricard (or Brasserie Zedel for slightly more casual but still very cool and French Art Deco). Or Quaglinios.
For chaotic fun and the best dim sum, pretty anywhere in Chinatown, particularly perhaps Dumplings Legend.
For the best view, any of the restaurants in the Shard