King George didn't have that. Instead, his problem was with "recycling" heme, not in producing it ~ which is common with "acute intermittent porphyria". Both conditions will definitely give you photoreactive urine ~ but "purple urine" is pretty much confined to "acute intermittent porphyria".
Of note, it can come out with a normal color, but sit it out in full sunlight and it'll change color. George III's courtiers noticed the color change because most palaces in those days depended extensively on natural lighting during daylight hours and, as it turns out, kings did all sorts of things in front of courtiers back then.