Auto-replies live on the server, so they go out even when your computer is off.
In webmail: log in at https://yourdomain.co.uk:2096, open Settings → Autoresponder (or Responses, depending on the view), turn it on, write your message and set the start and end dates.
In cPanel: Email → Autoresponders → Add Autoresponder. Choose the address, write the subject and message, and set the interval (how many hours before the same sender gets the reply again — 24 is sensible) and an optional start and stop time. It switches itself off at the stop time, so you can set it before you leave and forget about it.
Keep the message plain: who to contact in the meantime and when you're back. Avoid including anything sensitive, because the auto-reply goes to whoever emails you — including newsletters and, occasionally, spammers.
