Hey @WillSee, thanks for checking in with us.
Since you joined a Business team, only the admin is able to convert your account back to a Basic one again, as they have the capability to manage the team.
You can reach out though to our Support team from this link, if you'd like some further assistance on the matter.
Perhaps in the meantime, you can setup a new Dropbox account and share your files there.
Let me know if you have any additional questions though and I'll make sure to get back to you as soon as possible. Cheers!