Most people think of the VAT threshold as an annual figure. It is not. HMRC tests your taxable turnover on a rolling twelve-month basis — the total of the last twelve months at the end of every month — which is why a busy season can tip you over long before your financial year ends.
There is also a second, forward-looking test: if you expect to cross the threshold in the next thirty days alone, you must register straight away. Miss either test and registration is backdated, so you can owe VAT on sales where you never charged it.