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But you'd design for 200mph, given that >150mph is a normal feature of Scottish weather, i.e you'd expect it to happen multiple times during the life of a wind turbine in that location.

150mph is not a 'normal feature of Scottish weather'.

Turbines are designed to shut down in winds over 25m/s (~55mph), which in a 25 year wind speed distribution of a pretty windy place* in Scotland, amounts to a very few percent of the year. Winds over 100mph are significantly rare. Wind turbines are designed to withstand the 100yr extreme 3s wind gust - which can cause resonance in the tower or blades. Something clearly went wrong with that turbine - not necessarily caused by the wind, but it won't have helped.

*the location of Scotland's most powerful wind farm (at the time of it's construction), which had a mean annual wind speed of 10.9m/s.