Hmmm, that's interesting, so it seems the gain to 'us' is very limited, and the whole exercise is therefore for Zen's benefit (cost cutting perhaps)? That puts an interesting slant on things............
There's the potential for better gains depending on how different the route into Zen's core network is between their own backhaul and BTW. In my case, both connect to Zen in London, hence the miniscule difference.
I imagine if you're further north, then if Zen's backhaul goes to Manchester instead of London, you could get reduced latency to Manchester based services.
I have no idea if this setup actually exists though, it's just me speculating.
On the business side, once you get to a certain scale, i imagine it becomes cheaper and simpler to cut out the middle man in theory...
Unless you end up with unforseen issues 😬