What @gg30340 has said is correct, if you delete one of your 11 email address this should free up space to create a new one. I am sorry if this hasn't happened in your case. I'll be able to take a look at this for you. Please can you send me in your details using the "Contact The Mods" link found in my profile.
This arrived today. Hello gspearson,
Your topic recently received a reply.
Topic: Only 11 addresses Date: 18-03-2016 15h40
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I raised the issue here: https://community.bt.com/t5/Email/Deleted-BT-Mail-addresses-not-fully-deleted/td-p/1551615
On BT Mail, deleted addresses count towards the user's quota even several months after they are deleted by the user.
Let us know how you get on.
Good. Hopefully it will be fixed for all users in future. However, I suspect they will just free up the user quota by clearing out deleted addresses on a per user basis on request.