Why would we have cared what Britain was doing?
They were our cold enemy during the Civil War. (At least, they were the cold enemy if you were on the Union side.)
What the rest of the world was doing was irrelevant to how the 13th was drafted. The war would be won. Slaves would be freed. The only questions we were considering was how to put that all down on paper. And for that question, the only thing that mattered was domestic politics.
The "loophole" is, to put it in modern terms, a concession to the American penal-labor-colony industry lobby. (Well, not really, since we didn't have private prisons back then. All the work being done by US penal labor colonies was directly to the productive benefit of the US government.)
In fact, even a review of the Congressional Record, which I had the misfortune of having to do on these very two questions for a Capstone project back in undergrad, makes that very clear. It's obviously been a very long time, and the material involved was voluminous, but I can't recall offhand any mention of the penal labor industry. The debate was dominated by appeals to interests and even feelings of slave states.
Imagine if Britain had been the one drafting the 13th amendment, at the same point in history. Removing this language would have meant giving up on the penal labor colony known as Australia. America was—not as famously—doing exactly the same thing.