Hi,
If you are in the UK and supplying digital services to other businesses and these businesses are all outside the UK then these supplies would be outside the scope of VAT and there would be no obligation to register for VAT.
You can register for VAT in the UK voluntarily however.
You can see guidance here:
Working out whether you need to register
The place of supply of digital services
For Self Assessment, yes, it must be for the UK tax year. you will also need to amend your accounting period going forward for Making Tax Digital. You can see guidance here:
Making Tax Digital for Income Tax
Thank you.