A campaign of non-cooperation by individual farmers could include:
- never filling in official surveys, etc.
- never being available when officials call (either in person or by phone)
- not submitting paperwork such as movement forms
- closing down the countryside, including every footpath, etc. across farming properties
Sure some of it's illegal and some sailing pretty close to the wind, but such a campaign with a large percentage, or indeed majority, of farmers and sympathetic landowners joining in would be very hard to police.