They most likely went from the port of Libau.  There was a rail line that went from Dvinsk to Riga. That trip took about 5 hours. There was another rail line that went from Riga to Libau. (Or they might have gone by wagon- that part was a short trip.) From there they took a ship to England. There are sites online showing old rail routes in Europe. (I'm sorry but unfortunately I don't have access to my notes for the next few days-I am in transit-otherwise I would be able to give you more info.) But if you Google this you should be able to find the maps of the rail lines.

Although travel from Dvinsk to the ports in Germany was possible - there were border crossings into Germany, which required various documents making crossing
difficult. Libau was within the same country. There were no impediments to travel. 

Judy Winstan Brandspigel
Montreal, Canada

Researching  BLACHER, VINOKUR, KATSEFF  from Dvinsk