Caption should read: “That moment you forget the UK had slaves before the US existed.”
Not a difficult thing to forget. Practically everyone had slaves before the US existed.
Although, in fairness, the Normans did make slavery in England practically non-existent by about 1200. Later colonies and the latter slave trade were very different things of course, but the Royal Navy were an immense force for good in the eradication of the slave trade in the 19th century.