We don't design our own because we can't, the only British designer left (sort of) has never designed a successful helicopter! I cannot even think of a truly successful design to come out of that factory; maybe the Lysander but even that was a fundamental failure - it was designed to be a light bomber.
Buy the 18 straight of the shelf - completely cut out the middle man and get the contract fundamentally right first time. Personally, I thought the Blackhawk worked and looked just fine long before Westlands got their hands on one.