Sparkus seems to have is sussed. I'm less confident to ask to see the girl before
66 Middlesex Street isn't subtle in anyway - but the goods are on display. The other places (that I've been to) you don't get to see who you get until they enter the room - so if you're fussy you'll need to ask.
Other places that I know of (but can't comment on all) are in Toynbee Street (Wentworth Street end), Whites Row, Middlesex Street (at Liverpool Street end), New Inn Yard (Shoreditch), Gunthorpe Street and Artillery Lane. I'm sure there are other places that I've missed. I'll leave it for others to correct me