There are many willow merchants in the Uk, but more willow growers. GM, GN and others (including Wrights) will buy trees from landowners who have willow trees on their land. They'll also plant willow trees on landowners land, with a promise of a buy back when they mature. This is an ongoing process, which has already been going on for ages. Hence the constant supply. The demand is outstripping supply currently, hence the fight for willow and increase in price. Nobody just solely has fields of willow of their own, where they just have a revolving supply of willow and can therefore dictate the market.