There's plenty of quiet bars in and around soho. They tend not to serve obvious commerical brand drinks like Fosters/Guiness etc etc but their own less known equivliants. They tend to be frequented by people that work in the local area rather than tourists. They aren't too good for picking up usually as they tend to be full of insular groups of co workers that are trying to unwinde after having worked long hours but they are great for dates and quiet drinks once you've picked someone up from some place else.
Also there are plenty of quiet gay bars, and taking a girl to one of these can be a blast too and should give you plenty to talk about*
*Provided you are secure enough to chat up a girl while a pair of dudes rub stubble in the corner behind her of course...