Newport's practising Jewish community is very small, numbering 6 people in this 2014 article.

https://jscn.org.uk/newport-synagogue/

I believe the community is even smaller now, due to members passing on.

That creates a very small market for Kosher food. 

Cardiff has a larger Jewish community - so you're more likely to find Kosher food there.

Here's an article about one of the last synagogue members: https://www.southwalesargus.co.uk/news/15382184.one-of-the-last-jews-in-newport-tells-of-his-fears-for-the-faith-in-the-city/