Part of any Defence procurement decision is the Industrial Impact. This is (or was) put together by the DTI or whatever it's called today and comes in at Ministerial level, at a relatively late stage. Politicians tend to listen to these more than the military because these directly involve jobs, votes and political heat.
It is always very hard to make a military case for an MOTS item if a UK company is saying 'We can make that for the same price' (or a bit more if loads of jobs are claimed to be at stake). Good Project Management, sensible procurement strategies and everything else that leads to a successful buy are, at this point, poor politics.
In the case of CH47 vs EHI 01 it came down to jobs in Yeovil and down the Yeovil supply chain so the RAF was told there had to be a mixed buy.