Back to the original question... Yew, if you can get that in England. I know from the History Channel that yew from southern France was the favorite material for Welsh longbows. The core of the tree and its outer wood have very different properties. I forget which is which, but one is very springy and the other is rather rigid. This gives you the advantage of lamination without having to laminate.
I was gonna say yew, but they beat me to it. I did see some interesting plans for a crossbow made from the leaf spring of a car, called the "atom smasher", of course it was from the 1950's.
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I used to know someone who had built a crossbow from a leaf spring. 300 lb. pull, with crank and all. Very authentic. However, he could not shoot standard crossbow quarrels. They would shatter. He used 1 in. solid oak dowels to make his bolts.