Hunting a wild animal with dogs is illegal in England, Wales and Scotland.  If you witness illegal hunting there are a number of things you can do:

1. If you have a camera of any kind (even on a mobile phone!) try to capture images of the incident, its aftermath or the people involved.  Be careful though!
2. Make a note of the date,time and location of the incident.
3. Make a note of any vehicle registrations that are connected with the incident.  E.g. if you saw a fox killed by a pack of hounds and the body was picked up by somebody in a Landrover.
4. As soon as you can, write down what you saw.
5. Report the incident to the police and ask for a Crime Reference number.  Make a note of the name of the officer who received the report.
6. If the incident happened on public land, put in a formal complaint about illegal activities taking place. Hunts may only use Forestry Commission or MOD land under a strict licence.  Breaching that licence can mean they are no longer allowed to use that land.  Similarly, charitable bodies such as the National Trust, Woodland Trust or local charitable trusts or parish councils will take a dim view of law breaking on their land and may remove any "sporting rights" they have given the hunt.
7. Inform your local paper, radio station and/or TV station.  They will be especially interested if the land in question is public land, a local beauty spot or the police are investigating.  If you do not want to have your name mentioned in any report or your photo taken make sure you tell them.  They will usually quote you but state you did not want to give your name for fear of reprisals.