What might be the cause for a head light at night to only operate on high beam ?

 

burnt out bulb?
burnt out fuse?

 

Burnt out bulb. You have separate low beam bulbs and high beam bulbs. The low beam is the very common H11 size bulb. It is cheap and very easy to replace. Every auto parts store or Amazon has them. You may want to replace both as they have equal wear and are probably original.
Just do not touch the bulb glass with your bare finger as the oil will create a hot spot leading the early bulb death.

You did not state if it was both bulbs or one. The above assumes one bulb. If both, check fuses and if okay, then it may be the headlight stalk switch has worn out.

 

Easily replaced H11? I think this is true if OP has incandescent lamps, but not if the car has the LED main beams installed. Wasn't that sold in North America as an option?

note, the high beams (the two bulbs closer to the centreline of the car) were always incandescent from the factory.

 

Very few had the LED headlight option. It was probably part of the Ultra Premium package with heads up display, etc