Oh, this is fascinating! I hadn't really thought about Biggles's conception of Sakhalin, but he clearly is thinking of an ordinary prison--he only really learns more about how awful it is after he starts committing to the plan. And I think he would also reflexively assume a kind of honor and decency is it play here even when it's not, and even when he doesn't respect the Soviets; he instinctively uses his own moral yardstick for comparison and so obviously doesn't hit on "hellish forced labor."