Speculation: Quite a few photo website contain nudity, shock horror that nudes are actually a genre of legitimate photography. So the OSA is meant to protect kids from pornography (erotic image) which many of the nude photos don't really fall in to as nude photography focuses on the human figure, lighting, shadow etc.
So maybe they are blocking the UK so people elsewhere can still share their artistic images without falling foul of the law or requiring moderators to remove them.
I have not used imagur. On here, I have set my feed up to hide all images, in addition the tagging for naturist related images are very comprehensive it is blatently obious what the photo contains.
Meanwhile instagram et al are STILL allowing self harm and suicide posts in to feeds of under 18s, I am sure META has a much bigger budget to help deal with this than imagur.