Sal mentioned at a show in the UK that a member of the Spyderco team suggested they call the UKPK the same asthe rest of their knives that don't lock. FaultyImagine all the warranty service requests, "there is something wrong with my new knife, the locking mechanism is broken."
Having always used non locking knives (mostly SAKs) I've never had an issue with it but I do think OHO is much easier with locking folders.
From what I gather the William Henry legacy didn't go down too well in the US. Meanwhile I arranged a group buy of over 25 of them in a single go and many more have been seen on the British forum