download manager asks me for a userid and password for the download site.
What do you mean with "download manager" ? Are you using a 3rd party download manager ? Have you tried disabling it ?
There's no authorization needed to download any of our files. The issue is surely on your side, and I think it's a problem of your ISP.
We host our files on Mediafire, and we have a proper and legit, business account with them, which allows us to offer our downloads to everybody with high speed (since they have multiple servers and each file is stored on a different one) without requiring users needing to have a Mediafire account themselves.
However, it looks like you are in the UK, and we got word from some users, that some ISP in the UK have started to block the WHOLE Mediafire domain because, instead of doing the proper effort to block specific sites and files (which cost time and money), they blindly trust a blacklist set in place by the IWF, so they might block the *whole* of Mediafire, because some might have shared questionable content on it.
See here from this thread about BT:
https://community.bt.com/t5/BT-Infinity-Speed-Connection/BT-amp-IWF-Stops-MediaFire-com-links/td-p/534832