Hi,
I'm a UK student in Year 13 and I'm applying for Economics at St. John's College.
I have also been worrying about the interview invitations, and so I called up the Admissions Office. The information I received was specifically for Economics, but I expect there is a similar case with other subjects. The polite staff told me that, because of internal disagreements on the number of applicants the college could interview, the college has been slow in sending out invites. They told me that they intend to send the invites for Economics interviews out to applicants on Friday. However, I do not know whether this is by post, by e-mail or by the portal.
I hope this has helped to put some minds at ease, and good luck with your interviews and your other university applications.