Registering for Self Assessment while resident abroad

I'm trying to find clear guidance on this but can't find anything. We moved to Japan in October last year and I need to declare some rental income that received after moving. The guidance online seems to indicate that I cannot file online, but need to send the return via post. However, the form to "Register for Self Assessment" has a mandatory UK address field. I have an address for mail purposes in the UK (i.e., my parents house) but my primary address in reality is in Tokyo. Is it just the case that you don't therefore need to register online if you cannot apply online, and I need to call to request the paper forms?

Hi
If you have a personal government gateway user ID and password or you once you have registered for one, you can complete an online tax return.  
You will need to purchase some 3rd party commercial software to do this.
 You would complete the 3rd party tax return, which includes the residence section and submit using the user government gateway ID and password.  
You can find guidance on registering at:
File your Self Assessment tax return online
You can find a list of commerical software suppliers at :
Self Assessment commercial software suppliers
Thankyou.