Are you allowed to turn around on a motorway by driving through a gap in the central barrier?

No, on all motorways, there are turning areas at the side of the road, as well as gaps in the central barrier, at regular intervals – these are only to be used by rescue, police, salvage and road maintenance vehicles.

If you need to turn around on a motorway, you should take the first regular exit and then drive back onto the motorway using the nearest entry slip road.