Are you allowed in this situation to keep whichever distance from the oncoming lorry you are passing?

No, the distance from an oncoming vehicle cannot be arbitrary – it must be safe. According to Art. 23 para. 1 of the Road Traffic Act, the driver is obliged to maintain a safe distance. When passing a large heavy goods vehicle, you must be particularly cautious and maintain a greater lateral distance due to its size and possible air turbulence.