I would say your concerns are general to Zen's network rather than their Openreach FTTP packages.
Yes, that is a fair assessment.
If you get routed to Manchester, you can just ask them to change it to London. Happened to me, latency of 25ms compared to ~10ms with the previous provider (on VDSL2). I can't remember their reason for it to be routed this way, but after the change to London I was seeing the usual ~10ms.
Good to hear they're flexible enough to do such things.
However, sometimes I find it can be as high as ~20ms even now. Just the luck of the draw when getting a new PPP session, but I don't notice any difference until I see the figures.
The last time I had "luck of the draw" PPP sessions was around the time of Pre-BT / early BT days Plusnet. In the last 8 or so years I've done a dance around BT / Plusnet / EE and back again (and repeat) serially abusing their various cashback offers to minimize cost. Always around 7-8ms mark, rarely noted anything untoward (there were a couple of bouts of peak congestion nastiness just recently - I guess peak COVID - but other than that, and a line fault which was sorted, it's been good as gold)
Nervousness about prematurely losing my copper pathway and number though sees me reluctant to stick with BT/EE/Plusnet.
In regards to the "fluid" download speeds, I find that applies to the single-threaded downloads. For example, I might be able to get 300 Mbps throughput with multi-threaded downloads, but single-threaded downloads may be approx. half. Fortunately, most of my downloads are multi-threaded - and some of the mirrors I use do not always have enough bandwidth to saturate my connection anyway.
I'm not so bothered about flat out speed per se - to an extent 900/115 is a bit of an indulgence in tech for tech sake (everyone wants a nice fat test result though, right?!
) more about the extent of the variation and whether it is common to all on 900/115 services, or something Zen in particular are finding.
Of course this could equally apply to any network provider, I'm just singling out Zen as I've got an order in at the mo and if I got to the point of thinking better of it I could always pull the parachute on the cooling off.
This is my current EE FTTC 80/20 home connection (I run with smart queues in the router, without it would be much hairier):
This is what my Essensys serviced office connection across the road from my house looks like (it's a big fat fibre link in, and I'm on a shared 80/80 tier within that with own static IP, though I don't think I'm actually sharing with anyone from what I can tell). Again, smart queues enabled. Probably will have to upgrade the USG3 router once home FTTP is in, as it isn't fast enough to VPN at the higher rates.
I guess you can see why I'd treat anything resulting in much worse than these kind of results with utmost suspicion, given there's clearly no geographic limitations here.