Your streams seem faster on some services than others. Same resolution. Same bitrate. Same device.
The difference might be HTTP/3 — a newer protocol that reduces connection overhead.
A modern British IPTV reseller supports HTTP/3 (QUIC) on their CDN. Streams start faster. Network changes (WiFi to cellular) are smoother. Packet loss has less impact.
A legacy reseller uses HTTP/1.1 or HTTP/2. Connection establishment takes longer. Network handovers cause re-buffering. Packet loss kills performance.
I tested HTTP/3 vs HTTP/2 on the same network with simulated 2% packet loss. HTTP/3 maintained quality. HTTP/2 stuttered repeatedly. The only difference was the protocol version.
A forward-looking British IPTV service keeps their CDN configuration current. They enable HTTP/3 where available. They know that better protocols make better experiences.
Here's a test you can run. Use a browser's developer tools to check the protocol version of stream requests. HTTP/3 (or h3) indicates modern configuration. HTTP/2 or HTTP/1.1 indicates older setup.
The technically current IPTV reseller UK will be on HTTP/3 or actively migrating. They care about the underlying transport because they know it affects your experience. That technical depth correlates with overall quality.