Got a reply back from my ISP (Cuckoo Broadband), they say some loss is to be expected and consider 5-10% to be a high amount of packet loss (I assume they mean for FTTC).
Thing is, the impression I get from the Stadia subreddit is that even 1% is going to cause problems (many seem to have much better FTTC or FTTP lines). It's something I've had issues with on other game streaming platforms too.
Maybe it's only something that will affect game streaming? Would 1-2% loss have much impact on video calling software?
The interesting thing is, I (sort of) corrected the packet loss issue when I was using the Asus DSL-N16 router/modem, but only by limiting the upstream sync speed to 1mbps, which causes other performance issues...