All entities that are sent a notice to deliver a corporation tax return are required to file that return online. Also, any organisation that is within the charge to corporation tax must pay that tax (and any related payments, such as interest on tax paid late) electronically.
The requirement to file the return online includes clubs and smaller charities and the like. However, in those cases where an organisation is not required to submit its accounts to Companies House HMRC will allow them to attach their accounts to the return submission as a PDF file rather than provide the accounts using iXBRL if they prefer to do it that way. HMRC do still require the computations from everyone to be sent using iXBRL though.
There are two situations where a company or unincorporated organisation may be exempt from online filing: