When you import from, or export to, countries in the EU, you generally don’t have to worry about VAT or customs duties. That may change when the UK leaves the EU at 11pm on 29 March 2019.
It’s possible that the UK will leave the EU automatically by operation of the law (having triggered Article 50) with no withdrawal agreement in place. In that case the UK will immediately be treated as a third country in relation to the EU for all trading purposes, including for customs duties and VAT.
For imports, the VAT will have to be paid at the border before the goods can enter the UK. Similarly, your EU customers will have to pay VAT at the border when they buy goods from your company which is based in the UK.
Exporting is more complicated. For example, to ship goods into the EU your business will need an EORI number, and the commodity code for the goods. You may also need a special licence to move the goods, particularly for food or animal products, and tariffs may be imposed under world trade organisation (WTO) rules.
If all your overseas business is currently done with customers or suppliers in other EU countries, you will need to quickly get to grips with VAT on imports and exports and the customs procedures required. HMRC has recently written to businesses in your position, with advice on where to look for guidance on those issues. There are nine detailed Gorvernment guides on importing and exporting procedures that will come into force if there is no deal on the withdrawal from the EU which you can read here: https://tinyurl.com/NodealBRImEx