The idea came, was developed in a couple of minutes, and then I parked it under the heading "good ideas that will never happen."
Then, browsing the HMGov e-petitions site, I came across this petition: Create Wildlife River Buffer Zones. It amounts to the same thing, except promises compensation for farmers (I'm not sure about that). Anyway, I signed it, of course.
The threshold for petitions to generate a debate in Parliament is 100,000 signatories. When I signed up to the Create Wildlife River Buffer Zones petition, it had a grand total of 10 signatories. Presumably it now has 11.
Still, it doesn't close till next August...
Update 23.iv.2012: 27 signatories. It's not looking good.