I don't think HoD do walk-ins at their London places, only pre-booked appointments.
I can't think of anywhere in London you can just walk in and do a selection, as per Northern parlours...perhaps it ain't so grim up North after all!!
There are the Soho walk-ups, but only one girl per flat, so you would have to visit a few and turn girls down if you're looking for a certain type. They're cheap, but it can often be a rushed, impersonal service with an unenthusiastic girl.
I would save your cash for the VFM options back home, unless you want to splash out on something exotic you can't get in Manchester, go by the reviews here.
I know some guys travel down from Northern parts to attend The Phoenix Club parties, perhaps that would be something different.