Well, LON to ASI via the great circle direct route wouldn't have been possible due to African overflight. So just nipping the tip of Africa via Gran Canaria and Dakar would mean distance of 3746 nm on a route similar to this:
FAI average speed equates to 445.114 kts, giving a flight time of 8:25. Add to that the requirement for island holding fuel and I'm still not sure how John managed it without AAR....
SW Apps = South West Approaches. Probably no further out into the Atlantic than 15°W?