It would have been more correct to say that the sinking of the Lancastria represents the largest loss of life from a British maritime incident.
Possibly, a British military engagement might fit as a descriptor, but I would think there were British battles where more than 4,000 men were killed, maybe even on British soil, with all their civil wars, but I really don’t know.
Plenty of battles where we have lost 4000+. From civil wars through the battles of Empire, to both world wars: the Somme’s first day is the worst, 20000 dead.