The Bulleid diesels had different power arrangements as EE managed to increase output as they were developed. The eventual Class 40 took elements of both the SR and LMS prototypes. Whether any production diesels would have used the EE nose, or retained the flat front is debateable.
I would imagine a lower power diesel, say Bo-Bo 1200hp would have been used - perhaps in multiple from Waterloo, with each then taking a portion to North Devon/North Cornwall?