That's because their main commercial partner (both because of geographical proximity and shared language which allows using the same packaging) isn't fully metric.

It used to be the same in Canada (or at least Québec) but I've heard that more products are sold in round metric quantities these days.

That's true for some sausages and jams, but all butter is sold in multiple of 250g in Britain.

https://www.tesco.com/groceries/en-GB/shop/fresh-food/milk-b...

Nearly all the butter you see here is labeled Irish, as is most of the other dairy and meat. The north has the opposite, most of the meat/dairy is labeled from NI. (With the exception of some of the branded stuff, like kerrygold butter which is available on both sides of the border. )

Even for TV size?