VAT is a separate tax and once your turnover exceeds £85,000 in a year you will need to register for VAT whether you are self-employed or Limited Company.  If you do want to register for VAT you will need government gateway account with HMRC you can find in the link:

 

https://www.gov.uk/log-in-register-hmrc-online-services/register

 

If you exceeded the VAT threshold in the past 12 months

You must register if, by the end of any month, your total VAT taxable turnover for the last 12 months was over £85,000.

 

You have to register within 30 days of the end of the month when you went over the threshold. Your effective date of registration is the first day of the second month after you go over the threshold.

For Example: Between 10 July 2017 and 9 July 2018 your VAT taxable turnover was £100,000. That’s the first time it has gone over the VAT threshold. You must register by 30 August 2018. Your effective date of registration is 1 September 2018.