UK broadcasts at 50fps. US broadcasts at 60fps. Your British IPTV may need to convert between them.
Here's the technical detail. A British IPTV reseller can pass through original frame rates (50fps for UK channels, 60fps for US) or convert everything to 50fps or 60fps. Conversion can cause judder.
What actually works for smooth motion is matching frame rate to your display. UK viewers with 50Hz displays want 50fps. Converting 60fps US content to 50fps introduces judder.
I watched US sports on my British IPTV and noticed slight judder. The reseller was converting 60fps to 50fps. Switched to a reseller who passes through original frame rates. Judder disappeared.
The pattern that keeps showing up is that IPTV reseller UK operators who passthrough original frame rates preserve smooth motion. Those who convert for compatibility may introduce judder for purists.
Honestly, if you notice judder on US channels, ask your IPTV reseller UK if they convert frame rates. "Passthrough" is better for motion. "Converted to 50fps" explains the judder.
That said, most viewers don't notice frame rate judder. Sports fans on large screens are most sensitive.
In most cases, British IPTV frame rate conversion matters only for discerning viewers with large displays.