SamB wrote:

Is there anything stopping one going to a station of the
worst-performing TOC (not that I know what that is), and getting a
zonal season ticket, even if one doesn't travel with that TOC?

As far as I'm aware, there's no impediment that prevents one from so
doing, and indeed I believe some savvy passengers do actually take
advantage of this.

Bear in mind that it only applies to renewals of monthly or longer
season tickets.

Also I'm not an expert on this - so anyone who is please do correct me
- but I understand that the charter discounts are calculated according
to performance on particular routes/groups of routes as opposed to
across the TOC (and a season ticket will thus 'belong' in one route
group or the other).

I also think it's pretty uncommon for a TOC to have to offer a 10%
charter discount for very poor performance, but the 5% is quite common.
Of course it may be the case that the TOC performs better the next
quarter, so they no longer have to offer a charter discount. However
they may (and often do) slip back and have to start offering the
charter discounts again.