Two bits of speculation should now just be left behind. The fuel has already been dealt with; it is cleared to operate with any mix of avgas and jet A1, no flight manual warnings about operations using avgas except the 150 hour limit adjusted proportionally to the percentage avgas.
Second this aircraft is perfectly capable on one engine. Although I operated King Airs under performance class B my type rating sim training was to performance class A standards. Of course Stapleford is going to be nowhere near long enough for that, but it is not a poor performer on one once the engine is feathered. With little load it should be capable of over 500 feet per minute.