Hi hacer isik goktas,
If you are selling digital services from overseas to individuals in the UK then you will be making taxable supplies in the UK and there is no threshold to reach before the requirememnt to register for VAT in the UK.
You would therefore have to register upon your first supply to the UK.
Please check the guidance to confirm that you are a Non Established Taxable Person and that your supplies are condidered 'digital' services.
Please see the guidance below:
9. Non-established-taxable-persons (NETPs) — basic information
Defining digital services
Thank you.