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Absolutely true, but farmers would need some form of positive incentive to persuade them not to grow crops or grub hedges and trees out to make more efficient use of their land. They operate on a £/acre basis and if any scheme awards them less than they could gain by 'improvements' they will improve away. The logical extension of this is that only the poorest land will ever be set aside.
Rather a lot, but all grouse are wild birds, not reared and released. Pheasants and red legged partridge are are very different matter.
The one problem with banning game bird release is that without it a lot more countryside would become plains of monoculture separated by barbed wire. Woodlands, hedges, headlands and game crops don't just benefit gamebirds.