The 12.5 K mileage (or 1 year whichever comes first) and filter change intervals is for cars used in favourable conditions. Which is normally the case in the UK..
6000 mile interval is recommended in dusty conditions ,or very hot,or very cold weather. Or other 'harsh' usage. So some European countries opt on the safe side and recommend more frequent oil changes.
But only doing short journeys is also considered unfavourable conditions, justifying oil changes at 6K mile intervals. With cold starts a lot of moisture is produced that may never gets hot enough to evaporate,. It can mix with the oil. Normally such users dont exceed 6k miles a year anyway, but should not be tempted to get their full 12.5 K miles 'moneys worth ' by extending oil change intervals beyond 12 months.