I'm very perturbed to see a named individual being publicly bad-mouthed for something that was no fault of his own - "I hope that this will haunt [the farmer] for the rest of his days." - yes I'm sure it already will, without you rubbing it in. As Pampera points out in his/her comment, the facts do not point to the farmer having done anything wrong.
The argument about pedal cyclists riding at high speed is a weak one as well. Cyclists should be using off-road tracks in a way which gives proper regard to other people's safety (and on bridleways they have a legal obligation to give way to pedestrians and horse riders, see Countryside Act 1968, section 30), and if they do this then they shouldn't have a problem.
On the facts as reported, I think the coroner was 100% right to record this as misadventure, as to pin the blame on the farmer would have implied a duty of care to people doing all sorts of illegal things (just like if a joyrider stole your bike and you were then held responsible if they crashed). It's not reasonable to expect people to have to make places safe for activities that aren't allowed to happen there. There was clearly no intent to harm.