I've run into this problem myself. One important question to ask is:
Have you ever successfully connected to BBC mumble?
If the answer is yes the problem is that the mumble server "knows you" by the credentials / certificate you used before, which is different than the one you are now trying to connect with.
I am not certain but I believe nothing has to be setup or configured on the server to gain access for brand new users. If you answered no to the question and still cannot connect then there is another issue at play I am not aware of.
IIRC, there is almost no info about certificates on the "howto" page for joining our mumble. I remember the first time I connected I had questions about that as the mumble client provides a few choices about it and I didn't have a clue what the server expected or which of those choices were important.
Perhaps the info about certs, are you a new user, ever connected with mumble are just too much detail for the howto page. But this issue does seem to come up fairly frequently and should be resolved if possible.
One thing I just thought of is if I am correct that connecting to the BBC mumble server the very first time requires NO server side setup (fairly certain that is true), then it's a matter of removing your cert on you client and connecting. I looked everywhere to find where the freak'n mumble client stored the cert but failed to find it. In my last issue, on a Mac, I uninstalled / reinstalled the app and connected with a different name and successfully logged in without difficulty. The key points to remember is to use a new name unknown to the server and make sure the client is running as "brand new", no knowledge of you or BBC server.