AAC. MP3. AC3. E-AC3. Opus. Different channels use different audio codecs. Resellers may transcode or pass through.
Here's the technical detail. A British IPTV reseller can either pass through the source's audio codec (original quality) or transcode to a standard codec (compatibility, lower bitrate). Transcoding preserves compatibility but may reduce quality.
What actually works for viewers is matching codec to your audio equipment. AC3/E-AC3 (Dolby Digital) works with surround sound systems. AAC works with everything but may not support 5.1 surround.
I had British IPTV audio that wouldn't play on my soundbar. The reseller was using AAC. My soundbar expected AC3. Switched to a reseller who passed through AC3. Surround sound worked perfectly.
The pattern that keeps showing up is that IPTV reseller UK operators who passthrough original audio codecs preserve quality and surround sound. Those who transcode to AAC simplify compatibility but may break surround.
Honestly, if you have a surround sound system, ask your IPTV reseller UK what audio codec they use for 5.1 channels. "AC3 passthrough" is good. "Transcoded to AAC" may lose surround.
That said, most viewers use TV speakers or stereo soundbars. AAC is fine. Surround sound users are the exception.
In most cases, British IPTV audio codec choice matters only for home theater enthusiasts.