What can I do?
I could do lots of things, but I doubt anyone would want to pay for the hardware and time involved to rebuild networks.
The fact that test today was OK and as far as I am aware there was no congestion on the network the speed test runs on and that is 10 Gbps capable means probably just one of those vagueries of the Internet or a little congestion on a bit of Plusnet which others have mentioned recently.