Slow decline in Gig1 speeds and now getting <565 Mbps - the minimum download speed

Besides complaining to Virgin Media, is there anything I can do?

Here's proof from Virgin Media's own speedtest:

Here's a typical speedtest.net result:

Here's data over the past month showing a slow decline in speed (a raspberry pi runs a speedtest every couple of hours):

Normally the speeds max out the 1 GB/s network interface, here's data going back a year:

I rebooted the router at the end of July (no change to speed) and rebooted again at the start of September when I noticed the speeds dropping.

I have tested the speed on my main computer, directly cabled into the router, and on two separate raspberry pis (a 4 and a 5) with CAT6 cable.

There is nothing untoward in the router logs, these are the only critical entries:

[Mod - MAC addresses images removed]

Thanks for any advice you can offer to help diagnose/fix this issue.