RR Darts, like this:
From:
https://www.britairliners.org/airlin...testbeds&id=30
G-ALXN and G-AMDB were converted to Dart power and operated as freighters by BEA, providing them with some very valuable RR Dart experience prior to the Viscount entering service. See here for more:
http://aviationtrivia. bl*gsp*t .com/2010/03/while-vickers-viscount-and-its-rolls.html (fill in the blanks... remove the spaces).