Hello from me in South Lanarkshire
I've got Kune kunes too
As for willow - as far as I'm aware once it's established you can quite easily get 6ft of growth a year,(as long as it's not a weeping or dwarf variety) the cut and come again thing is called coppicing and can be done with pretty much any variety of tree (though I think willow's probably the fastest growing
) Your ash will make the best firewood ever and doesn't need to be seasoned for quite as long as other wood - good luck and let us know how you're getting on.
Karen x