I think you would be able to find a girl in Soho willing to do this, they get some very weird/kinky requests, so they would see this as very tame. But you need to find the right girl, as some won`t be happy about the COB, or for them to masturbate at the same time.
Also do you mind what the girl looks like? Depending on your type, I would try Nadia (Berwick St, due back soon) or Roxy at Greek St, they are both friendly and would possibly accommodate.
Or if you have the time, just go for a wander round the walk-ups, and keep asking until you get a result, I`m sure you`ll find one who will say yes. Good luck.