When Walter is sent to France he is depicted at a station with LNER signage. This is incorrect if he is leaving London to go to France since the LNER ran north out of London. The most likely way to get to the south coast ports would be on the Southern Railway or, less likely, on the GWR to Weymouth or Plymouth.
At the dance where the girls meet Walter there are numerous US flags displayed. The problem is they are the modern, post 1960, 50 star flag. Not the 1948 star flag seen in WW2.
While in Liverpool Nan goes out clubbing with some rugby lads, however when Jamie is running does the street trying to find her he is actually running down O'Connell Street in Dublin.