Do you have any historical data from before? For example, months following the time of install?
No, the two Zyxel's are recent purchases, so I've nothing from before. Plus, I only moved to BT on 20 September, and the A&A line was installed a week later.
When scanning through, I didn't see any mention of an engineer coming out to test the line. I didn't see any mention of ISPs running remote tests, either. Have you asked your ISP(s) to run remote tests? If not, request those tests and see whether an engineer visit is advisable. Obviously take note of the applicable costs of a visit if there is a problem in your property (such as with the mastersocket, filters, etc.).
I had engineers out shortly after the A&A line installation. Would have loved it to be a shiny new copper pair run into the flat, but they used an exisiting unused pair so I'm not sure of the pair quality. Anyway, they had to come out because I had no service whatsoever. They found it had been misjumpered at the PCP.
Around the same time, the BT line was dropping out, so a separate engineer visit for that happened, however, this guy was a PSTN engineer rather than broadband. He found a loose termination in the wiring closet but apart from that it was testing OK.
As for the solution, I don't know. If there is a fault then hopefully an engineer can resolve it. If you're really concerned about latency, you could look at a leased line (which can cost hundreds of £ per month... but I don't know how that works if you live in a flat), see if there are alternative suppliers for fibre, or look at FTTPoD or Openreach Community Fibre Partnerships (though the latter two are expensive and I have no idea how that works in Multi-Dwelling Units (MDUs) like flats).
Unfortunately I don't see any of those options working. I managed to get Virgin Media on board to cable our flats but when it came to signing the wayleave to allow them to, one neighbour (out of 50 flats) put a stop to the whole thing. I asked our building factor to ask them to reconsider, but they firmly denied. So that, to me, means it doesn't matter which provider I go to, I have a difficult neighbour who will block it. Virgin Media are in every street nearby (our flats were built after CableTel had cabled the area in the 90s) so I'm surprised our BT cabinet is busy.
My exchange was on the list for Openreach FTTP upgrade earlier this year but was pulled in April "due to unforseen operational obstacles" so that looks like it's years away.
CityFibre are close by, but they're not doing our area for FTTP yet.
I will be on VDSL for some time yet.