Hi N.L. So,
If you have or can obtain a government gateway user ID and password (you need to be able to verify your identity to get one) you can use it to submit a commercial tax return, that include the residence section instead.
There is a list of commercial supplierswho for a fee, will let you use their tax return to submit online:
Self Assessment commercial software suppliers
The HMRC version, cannot be used to submit a non resident tax return.
If you cannot verify your identity, then your only options are to submit a paper tax return or employ an agent to submit it for you.
The Self Assessment tax return must be submitted as a whole, either online or via a paper version.