The general rule for this is that the road should only be split when the middle man is at least 5m wide.

There has been a tendency for some of the newer editors to come in and split roads that has anything more than a white line in the middle. Very annoying!

As per the wiki, roads should generally not be split unless it is necessary. Splitting a road always makes things more complicated (both routing and map editing); but if the road is wide enough or the median is wide enough, then it will be necessary otherwise GPS tracks will be too far from the road segment, and in some cases splitting the road is necessary to provide correct routing (such as when there is a turning lane or U-turn lane that needs to be represented separately to give the correct instruction).

Regarding U-turns, I believe these are only actually illegal if the intersection has a U-turn Prohibited (R213) sign, but when the median is so narrow it makes a U-turn very awkward to perform even if it's not illegal.

However, I should note that it is possible to prevent these by turning the intersection into a "mapcat bowtie"; this involves bringing the roads to meet at a single junction (instead of two junctions or four junctions); that way you can prohibit the turns for the U-turns. The reason it is called a "bowtie" is that you need to taper the roads gradually so they meet the junction at the right angle; if the angle is too sharp, then the turn directions will be incorrect, as Waze uses the angle of the turn to determine what instruction to give.