I'm fairly sure you can slaughter them for your own consumption, the link is from the goverment.
Keeping chickens for meat
If you want to keep chickens for meat, you must be prepared to eventually slaughter them. The Humane Slaughter Association runs courses on how to kill chickens while keeping their distress to a minimum. If you don’t want to do the job yourself, you could contact a local slaughterhouse. An internet search of abattoirs in your area will turn up a few options.
If the meat is to sell to others then thata a different story, hope this helped you