I am in the same situation for Paris and still waiting for the instruction letter since 2 August. Most people book anyway, but do it further away, London has many appointments, so you make sure they have your physical file (at least à month away) I think that the files do not necessarily leave at the same time as the in transit status, that is my theory, that the physical files can leave in the next batch at times. The notification from the embassy is just to make sure they have your physical file. It is just nerve wracking again to wait for a receipt notification while your case has been ready for 10 days now ...