Thanks so much for this reply! Time for a confession and a story on my part.
I've had this problem across a number of different ISPs on Octotel, for the better part of a year, now in two different locations. I consider myself somewhat knowledgeable when it comes to networking and routers, having worked in the industry.
I have always used a Mikrotik HAp AC Lite to dial PPPoE on my link, and Unifi AP's to broadcast the WiFi network and manage everything nicely. I usually swear by Mikrotik's as the gateway, and have for years, to get proper control of the connection and run queues and limits etc to properly monitor and manage things.
After sitting with this issue the other day, it occurred to me that I have never tried a different router, in the somehow off-chance that this would make a difference. So, since you guys had generously sent me a free router, I thought, why not, let's give it a go.
Well, I can safely say I feel rather stupid now, as the issue is totally gone, from what I can tell. Full speed to whatever international destinations deem important and speedtest to. In fact all international traffic looks far far better than it did before.
So, not believing that this was even possible, I totally factory reset the Mikrotik, set up totally from scratch (no preset configuration at all) only just enough configuration to dial PPPoE, masquerade the traffic and give me DHCP. The problem was easily reproducible, the minute the Mikrotik was back as the gateway, terrible international speeds, and full speed with the free (Tenda) router that was provided.
I am attempting to get another Mikrotik to test with, to confirm my suspicion that this is Mikrotik related somehow, as I struggle to believe that the one I have is faulty, but I'm happy to be proved wrong.
So if anyone is struggling like I am, and is using a Mikrotik as the gateway, I would propose trying as I did, admitting that sometimes things don't make sense (to me anyway) and giving a totally different router brand a shot.
Thanks so much for the tentativeness on this issue, but it seems resolved!