You must register for VAT if your taxable turnover during the last 12 months is above the VAT registration threshold or if your taxable turnover is likely to exceed the threshold within the next 30 days alone.
Taxable turnover is not just the profit made by your business but includes all the money going through a business in the form of sales of goods and services which are VAT rated. Goods and services which are not taxable and are therefore exempt from VAT, are not included when calculating your taxable turnover.
If your taxable turnover is below the VAT registration threshold, you may still opt to register for VAT. The advantages of this include credibility for your business. And, if you are selling zero-rated goods but paying VAT on the goods and services you buy in, you will be able to claim back any VAT.
However, voluntary registration does mean you will then have the added burden of keeping proper, up-to-date VAT records, account for all output tax on all your taxable supplies and make regular VAT returns which can be time consuming and costly.