Hi Oujiseiyaku,
If you are a UK established business and only supply services where the place of supply is outside the UK, then you may voluntary register if those supplies would be taxable if supplied within the UK and you are incurring vat on purchases in the UK.
Please refer to notice 700/1 section 2.9
2. Basic principles 2.1 VAT — an introduction
For guidance on place of supply of services please refer to notice 741a
Thank you.