Wonder if anyone has tried growing EW in the US.
"a short mild winter so not really more than -5C at worst, a warm summer so not really more than 25C at max and a lack of high winds."
That sounds exactly like the weather in the pacific northwest ... plenty of water there as well
Hence why I mentioned Washington State and Oregon.
I have been to both places and the climate is perfect...ie: no snow, but lots of rain and cool summers.
South island New Zealand is the same.
In Australia the best conditions for growing English Willow is Tasmania. The weather seldom gets above 24 in summer and while it gets to 0 in winter, it never goes below that. Plenty of water and space to have plantations there - logging is Tassies #1 industry.
http://en.wikipedia.org/wiki/GunnsIt can be done, it just depends on whether it is commercially viable.