Just thought I’d post an update. Today, after waiting for the ID verification process to complete I received a message saying I was unable to open an account.
I received a message off Monzo on Facebook Messenger and they explained the issue was because I’d previously closed an account with them (over a year ago), but they need to re-open closed accounts. Past users cannot apply the standard way. Hopefully this helps some of the others that were having the same issues 