Alphabetical. Numerical by channel number. By category then alphabetical. By popularity. Resellers sort channels differently.
Here's the sorting logic. A British IPTV reseller can sort channels by: number (101,102,103), name (BBC One, BBC Two, ITV), category (Sports, News, Entertainment), or custom (manual ordering). Each has trade-offs.
What actually works for viewers is sorting that matches how you find channels. Number sorting works if you know channel numbers (Sky-style). Name sorting works if you search by name. Category sorting works if you browse.
I prefer number sorting because I've memorized British IPTV channel numbers (401 for Sky Sports). Alphabetical sorting broke my muscle memory. I asked my reseller if I could choose. He couldn't. I switched to a reseller with number sorting.
The pattern that keeps showing up is that IPTV reseller UK operators who offer multiple sorting options (user-selectable) are most flexible. Most offer only one sorting method.
Honestly, ask for a screenshot of the channel list before buying. If the sorting matches how you think (numbers, names, categories), good. If not, ask if sorting can be changed. A IPTV reseller UK with user-selectable sorting is rare but valuable.
That said, many players let you re-sort channels locally. Tivimate can sort by number, name, or custom. Use that.
In most cases, solve British IPTV sorting at the player level, not reseller level.