If I recall correctly, the UK had a substantial balance of payments crisis at the time and it was essential that a large component of the 'Fifty-fifty Phantom' (as it was nicknamed) was of UK origin. Hence the Spey.
The UK also tried to offset the cost by selling the Jetstream 3M to the US where it would have been the C-10A; that also flopped when Handley Page couldn't keep to its delivery dates. But at least the 3M wouldn't have had the awful Astazou engine, it would have had Garretts!