I believe you are right.
I don't have Zen at my exchange but I do have a Cardiff based company called Spectrum LLU and they say they can use their network to provide the FTTC. I have always thought the LLU backhaul was big enough to cope with it. I would like to think I was right there it must be standard GB links?
Sadly, Zen sales got back to me to say I'd be stuck with WBMC if I switch - although C&W have a presence, and Zen apparently use C&W backhaul, it seems that doesn't extend to them using C&W's backhaul here. I'm also curious why they downgraded back from WBC (where they at least 'owned' the traffic from the serving BT POP onwards, instead of relying on BT's national network all the way) - does anyone have any ideas?
Between the bad experiences with WBMC in the past and IPv6 still being a "trial" for now, I think I'll stay put with A&A a bit longer.
(Aside from the unlimited tariff, Zen's appeal for me was having their own backhaul network and peering, plus an Akamai node, closer to me than London; as it stands, all my traffic gets shipped all the way to London as PPPoE, then over peering or transit from there; being stuck with WBMC would seem to negate all of that except for having an on-network Akamai node.)
AFAICS Zen haven't stopped investing in backhaul yet though, have they? Which might at least hold out the prospect of more exchanges being added to the list in future...