I followed all of these steps on DLP 15.5, saw the new locale - English (United Kingdom) - appear in the General Settings, selected it and then logged off/logged on again. The date format has not changed.
I restarted all of the services on the server again and that made no difference. Are there any further steps that need to be taken on 15.5?
Check your login User.. you will need to select the Local for EACH user.
PLEASE MARKED SOLVED WHEN POSSIBLE
Thanks, Ronak. Is there no way to make the UK locale the default? I thought that, by selecting it as the default in System > Settings > General, it would be the default locale for each user. Is this not the case?