Any 12 month period be it January to January, march to march or September to September.
Direct quote from HMRC.
Supplying goods or services within the UK. If your turnover of VAT taxable goods and services supplied within the UK for the previous 12 months is more than the current registration threshold of £77,000, or you expect it to go over that figure in the next 30 days alone, you must register for VAT.