As popcornpizza called out, most of the answers in this thread unfortunately seem unrelated to your question.

I live in the US and haven’t been to the UK in years, but I do visit Romania every year or two. I don’t notice a big difference between Coke Zero sold in the US versus Romania. I’m certainly not a Coke Zero expert, but they taste pretty much the same to me. My hunch is that Coke Zero tastes the same across Europe, but I’m happy to be corrected.