The wind and weather are so chilling that you cannot melt or rinse off the ice. You will only build up even more of it. Sailing south means deflecting the convoy from its mission. A delay in getting supplies to Britain might cost lives or even effect the entire course of the war. De-icer fluid doesn't exist yet, and it would take a huge amount to clear off a ship even once. The best course of action is to order the men to hitch up life lines and use axes to chop off as much ice as possible. The work is very dangerous, but it is necessary.

Convoy Duty: Iced Up - Clarence Mitchell

The Atlantic was an unforgiving sea for our small fighting ships. The ocean spray froze and coated ships with ice. It had to be chipped off so our ship would not become unstable and maybe roll over. Many hours I have spent chipping ice with my whole crew, officers and all, at the same time, with the threat of a U-boat attack. Our clothes would be wet, and we would be soaked to the skin. We would have icicles hanging from our eyebrows, nose, and chin. We would not break them off till we got below with a little heat. Some skin would come off also.