The VAT threshold is based on a rolling 12 months, not the tax year

Many people check their turnover against the VAT threshold once a year. HMRC actually looks at any rolling 12-month period. At the end of every month, add up your taxable turnover for the previous 12 months. If it goes over the threshold (currently £90,000), you normally have to register within 30 days of the end of that month. Late registration can mean paying VAT you never charged your customers. Check GOV.UK for the current threshold. If most of your customers are VAT-registered businesses, it can also be worth registering voluntarily, so you can reclaim VAT on your costs.