If you supply goods to a customer in a EU country you must charge VAT at the standard UK rate. However if that customer is VAT registered, as long as they provide you with their VAT number you can zero rate the supply. The customer's VAT number must be recorded on the invoice you provide to them.
When you zero rate supplies to an EC customer the value of the sale to each customer must be declared on a quarterly EC Sales List. You will need the VAT number of the customer to be able to complete this form.
Supplies of services to EC business customers must be charged at the standard rate of UK VAT and are therefore declared with other sales on your VAT return.
There is a limit to the value of goods you can sell to one EU country within a 12 month period before you have to become VAT registered in that country. This threshold is approximately 35,000 euros but varies from country to country.