It is true that the external wall DPC should be at least 150mm above finished ground level.
The key words are "should be" which is different from "must be"
The reason is this, as rain falls it splashes back off the ground and it is widely believed that this would be less than 150mm meaning the dpc would stop any splashes soaking the bricks above the dpc. Obviously primary purpose of the dpc is to prevent rising damp, but the 150mm is for splash back reasons.
Unless the wall is getting saturated, neither the NHBC or builder would do anything about it, even though it is not right.
Using a "trench" filled with drainage stones at 150mm below is often used as a cheap solution to the defect but it is not an alternative or approved.
Good news, from your photos, I see one airbrick. These are normally located on the dpc course so you may just have a dpc 150mm above finished ground level. That said, if the airbrick is for underfloor ventilation there should be more than one!
There are many posts on this forum about ground level dpcs. Quite why the housebuilders can't even get this simple rule right after decades is beyond m.