I don't think the gateway story is quite as simple as Manchester=higher throughput, higher latency; I think perhaps the best overall gateway for me seems to be London lo0-0.bng2.ixn-lon.zen.net.uk. The other three London ones are generally a bit worse. The Manchester ones perform faster than the two slower London ones for some tests into London. But Zen's own Speedtest server for me on any gateway is always well below par and Zen's own server via Manchester for me is the worst of all, often recording results below Zen's guarantee., which is useful I guess.
I did a bunch of testing again yesterday evening with the Fritzbox router "just to make sure" as Zen are insisting they need to get an Openreach engineer out to check the line (even though it makes line rate to some servers and some gateways). Seems fundamentally to be a waste of time to me because of being able to achieve line rate with their router sometimes (or with mine), just depending on where I end up or try to go, but what do I know.
Anyway, I guess I should be grateful it is a next step.
The Zen staffer said they'd prefer to work out what it is rather than just migrate me back as the plan is that the BTW links would be removed eventually and then the Zen one would be the only option. Which I can understand at a technical / operational / commercial level, but as an end user it's a bit of a bitter pill. The BT check of the line is the next step as all the remote checks they can do on the line check out with no fault.
Of course, there is the possibility that the Speedtest anomalies and the gateway variability is just a separate thing to the migration; but I can't shake the feeling that due to the coincidental timing one is exasperating the other.
It does look like separately the Ubiquiti UDM Pro SE only just / not quite manages to saturate the link when routing through it over PPPoE, though running the speedtests locally on the UDM over SSH to the internet does saturate the link (and has similar gateway dependent issues). It's a bit annoying that the PPPoE doesn't quite cut it, and I'm debating whether to hold onto the router as it's still within returns window. Though I otherwise like it, and am wondering if it is possible to use another device as a plain PPPoE "modem" and provide a DHCP IP based connection to the router, but without ending up doing double NAT, and it working seamlessly if Zen re-balance me onto a different gateway (ie the DHCP IP link then getting taken down and up gracefully etc). Has anyone set up such a thing in the past?