Actually it was NOT as hushed up as much as the stories go...I have a series of contemporary WW2 books called "The War in Pictures" They appear to have either been published in weekly parts by Odhams and bound after the war or are a bound edition from 1945-7.
There are several pages on the Lancastria, including photos and eyewitness accounts....
This two page spread states...
"Lancastria sunk by German Dive-Bombers.
While evacuating British troops from France, the 20,000-ton Cunard White Star Liner Lancastria - Well known to British holidaymakers as a cruising vessel - was bombed by Junkers dive bombers, set on fire and sunk off St Nazaire on 17 June. Nearly 2000 of the 5000 odd men on board mostly British soldiers and airmen were saved in spite of the fact that the ship went down within half an hour of the attack. The dramatic picture above taken from one the ships that rushed to the rescue shows men swimming for safety as she heels over preparatory to taking her death plunge"