Review of Irish and UK Files
Alan McCormick
The authors of the 2000 review pointed out that ;
"had the St. Phelim developed problems immediately after take off, it is unlikely that it would have continued on its way to London but rather would have returned and landed in Cork".
Your point is valid concerning identifying an aircraft 13 miles away. This distance is the equivalent of observing a commercial transatlanic airliner directly overhead flying at twice the normal altitude. At normal altitude, around 30 to 35 thousand ft, an observer would have difficulty knowing that there was an aircraft except for the presence of the contrail.