Have just double checked my Virgin (NTL) website and all working correctly. Even tried the
ftp://user.name:password@upload.ntlworld.com/ way of getting in and it worked.
Can only assume that as it is an ISP homepage it will only work from a Virgin (NTL) line. My guess is the other websites you can upload to are not ISP related. May well be wrong here. It certainly appears that unless you are using their network you cannot get access.