Might seem strange but have you tried moving the key around the area surrounding the ignition lock ? There is a built in immobiliser that needs to “see” the key. If you have a leather covered steering cowl that can be enough to stop the key and immobiliser talking to each other.
First time it happened to me was in my one week old GDi. It had been absolutely fine until I drove onto the King Harry Ferry in Cornwall. Drove on fine, turned off the engine, wouldn’t start on the other side of the river and I couldn’t get off the ferry! Won’t forget that one……
Nothing ever showed up on the OBD reader, battery turned the engine over quickly and was new. Happened on a number of occasions after but once you know the trick it normally works. Not always first time though!
Have had the same issue with my current car once as well so not restricted to GDi cars. Some cars seem to be affected some aren’t. Might be fine for years and then it will happen again. One of the many fascinating idiosyncrasies of our cars.
