You probably don't want primary sources. The primary sources -- wartime reports of shipping losses -- would be filled with inaccuracies and might be difficult to access or understand. Many ships were sunk while traveling alone, and may not have sent a distress call or may have misreported their position. In other cases, German U-boats sometimes reported sinkings that were later disproven. Researchers after the war combed through Allied and Axis claims for sunken merchant ships to reconcile these lists and created a much more accurate picture of shipping losses. Even those lists still have some contradictions and missing info, though.

While you certainly can examine just British-flagged merchant ships that were lost during the war, you'd be missing hundreds (or thousands) of ships from other nations that were also carrying supplies to Great Britain during the war.

An easy place to start would be the database on uboat.net, which includes not just the location of ships damaged/sunk (some ships survived attacks, but were written off after reaching port), but lots of other information. Their sources are listed here, which include some non-English language sources but is a decent place to start if you wanted to branch off on your own.