I am such a fool, I now remember - there is a super easy tool - the website
https://testmy.net/upload has an upload test where you can manually choose the size of the upload and it can be set to be absolutely massive so that it takes ages. It’s also fairly accurate if you don’t mind TCP as it doesn’t seem too overloaded - don’t know why the pipe is like or the server.
Cave : Be careful to change servers because it’s American and it can put uk users on a bizarre choice of US-based server, so you need to choose London in settings. There is also a ‘multithreaded’ option which might be well worth a look. Don’t know if that means multiple TCP connections, to saturate the pipe fully, otherwise why do I care about the internal design of their server app.