Paul Bailey went out of business. Strosek owners changed hands in england. And then somehow it split again. I remember talking to the owner of V12sales.com about it and he told me the story, but it's been so long I forgot the whole of the story.
But to answer the original question, Z1 buys from a vendor for the Strosek (or Strosek styled kit, whichever way you want to accept it) either here in the states or the UK. They don't make it in house obviously, so anyone interested in a kit should compare pricing with the Z1 and V12 sales. I imagine buying through Strosek in UK would be more costly or even not attainable depending on dealer only availability.