In air navigation, in the simplest terms, the point of no return (abbreviated as PNR) is the furthest point an aircraft can fly outbound while still having enough fuel to return to its base of origin. More formally, the calculation of the point of no return needs to also exclude the use of whatever mandatory reserves the aircraft must carry. The point of no return is not simply half the distance the aircraft can fly on its fuel load or half of the maximum range of the aircraft--it needs to take into account the effect of the wind, and therefore the forecasted ground speed of the aircraft outbound and inbound.
A more complicated formula for calculating the point of no return can take into account the existence of a diversion or alternate somewhere between the point of departure and the destination, in what is sometimes known as a "three point calculation of the PNR". In Biggles' context this hardly ever happens. He almost always departed from the only airport where he could get fuel and had to return there and nowhere else. In Biggles' Combined Operation, he refueled at Malta on the way to Venesos. On the inbound, he encountered adverse headwinds affected him such that he could not get back to Malta--in other words, he had flown past his point of no return getting to Venesos. He might have diverted e.g. to Athens but didn't want to do so because it might have caused political problems. In Biggles in Borneo, Biggles also flew past his point of no return on his outbound from Lucky Strike to Telapur because of unanticipated headwinds and he had to find fuel at his destination in order to return.