OK so there are instructions online about how to rate limit Chrome and Firefox from the developer console, but none of them seem to actually work for YouTube uploads. (Or I was doing it wrong which is also quite possible.)
The only working solution I found was a program called TMeter Free, using the documentation at https://www.howtogeek.com/347711/how-to-limit-any-applications-bandwidth-on-windows/
I’m happily uploading to YouTube at 2/3rds the max my ADSL can handle – ADSL becomes very slow on downloads if the upload is maxed out.
Tmeter is free from http://www.tmeter.ru/en/ – up to four processes only.