I have a book "Vickers A History" by J.D. Scott.
It says that the first two VC2 (Viscount) prototypes ordered by the Ministry of Supply were to be powered by the Armstrong Siddeley Mamba turbo prop engine. George Edwards, Chief Designer at Weybridge wanted the RR Dart.
Studies for BEA were carried out with other engines - 4 Merlins, 4 Twin Wasps, 2 Centuras, - but George Edwards pushed for the Dart and that's what it got.
As to the length of the nacelle, I think it may be so that all the engine and accessories are forward of the wing so that any failure would avoid damage to it.