Sabs do have form for using road kill foxes to "demonstrate" illegal hunting.
I don't know if the Bicester Hunt killed this fox, but as I said I don't think that video would stand up as proof.
It is a terrible law and is impossible to enforce. It is illegal to hunt and kill a fox using a pack of hounds. But it is not illegal to hunt and kill a fox using a pack of hounds so long as it was an accident. The police aren't interested so it is left to a bunch of what are essentially vigilantes, often using illegal activity themselves to prove that another group who say they are acting legitimately are in fact breaking the law.
Some drag hunts lay the scent on a horse's foot. I've done trail laying once. I thought my hard pulling horse would finally settle if he was right at the front laying the trail, but if anything he pulled even harder! He really lived to jump hedges 😁
I think there would be less likelihood of "accidental" fox hunting if the hounds were not trained using fox derived scent. Is it beyond the skill of experienced trail huntsmen to train the hounds to track an artificial scent?