Because I'm not signed up for online filing and doing so requires waiting for HMRC to post me a security code, I was about to download and fill in an offline CT600 (Short) for my small UK-registered company and send it to HMRC in the post, but I noticed on the text of the download page it states this:
Please check if you are required to submit this form online before downloading and sending it in the post.
Find out more about who must submit an online Company Tax Return
Unfortunately, the "find out more" link goes to a page that doesn't specify whether I have to do it online, but just has instructions for how to do it online. I've been wandering around the gov.uk web site and googling various terms for the last half hour, but have yet to find any definitive description of who has to file online and who is allowed to use the offline form. Can anyone give me a summary of what the rules are?