a jetlagged celebratory dinner?

Hello, all! I'm taking a trip to London the first week of October with my girlfriend, and I intend to propose to her on our first day there (a Sunday). I'm trying to decide on a place for us to have a celebratory dinner.

There is a twist, however: it will be our first day in London, and I will be severely jetlagged and probably have very little appetite. I would therefore be looking for something that's a bit lighter fare. Also, something not ostentatiously expensive (we'll get to those later in the trip), but that's still very memorable (preferrably, something distinctive, a place you wouldn't be able to confuse with a restaurant in New York or something - but that may be a bit too picky).

FYI, neither of us has been to London before, and we're both devoted foodies, with no dietary restrictions. We'll be staying near Euston Square Gardens, and restaurants we already have reservations for include Gynkhana, Restaurant Story, and Quality Chop House.

Any suggestions?

Thanks a bunch!