Most people are aware that once your annual turnover breaches the VAT registration threshold (currently £85,000), VAT registration is compulsory.
However, there are two further things to consider:
Firstly, it is the VATable (including zero rated) turnover in ANY 12-month period that should be considered, not just your accounts year.
Secondly, as well as your own turnover, you need to include the value of deemed supplies, for example where you receive services from an EU supplier which are accounted for under reverse charge.
For more details, please see //www.gov.uk/vat-registration/calculate-turnover
You can access the HMRC Quick Links page for VAT by clicking here