One thing to think about is if you want it to be more private or if having a throng of people around wouldn't bother you. London waterfront could be really nice, you could do it from the park across the way with her able to see the view of London, or you could even do it in that little off spot at Lombard's. When I had proposed, I had 5 places in mind (Bruce, the park, the rocks behind Mythos, the bridge at IOA, and Allah) with some backup ideas luckily the one I wanted eventually cleared out enough behind Richter's near Bruce. Either way it is going to be awesome just don't stress yourself.
For capturing the moment you can try to talk to one of the wandering photo connect people and maybe have them meet you there, I had planned to have one of them take a picture and get a caricature done afterwards, but alas the one time I couldn't find a picture taker. If you are near Lombard's you may have a pretty good chance since the big photoconnect area is right there near Disaster.