When it comes to VAT registration, an individual is required to register for VAT by 30 days after the end of the month in which her total taxable supplies for the previous 12 months exceeds the threshold of 85,000 and in doing so; the individual would get VAT registered on the 1st day of the 2nd month after the month in which the taxable supplies exceeded the threshold.
Keeping this in mind, if my taxable supplies increase the 85,000 threshold in 31 January 2021, I have to notify HMRC by 2nd March 2021 (30 days) and if I do, I would be treated as VAT registered from 1 March 2021 (1st day of the 2nd month after the month in which the taxable supplies exceeded the threshold).