Invoicing Foreign Clients
Unfortunately invoicing foreign clients for VAT is not that straightforward. There are different rules depending on whether the client is based inside the EU or not and whether they are registered for VAT themselves.
EU companies with a VAT number
Firstly you need to get hold of the company's VAT number. The easiest way to do this is to ask. Then look it up on the EU portal to make sure that it is valid. Invoices can then be prepared with a zero rate of TVA, but they must bear the words "Opération bénéficiant du régime des livraisons intracommunautaires (autoliquidation) - article 138 de la directive 2006/112/CE" and the client's VAT number.
EU companies without a VAT number
If you are selling a "dematerialised service" you have to charge VAT at the rate that is applicable in the client's country. For client's based in the UK you will have to charge 20%. On the invoice you will need to state that you have invoiced at the UK rate in accordance with EU regulations.
You will also need to declare them through the MOSS portal on your company's account on the Impots website. You have to register for this separately from the main VAT declaration and you need to complete a quarterly return, by the 20th of the month following the end of the quarter whether or not you have had any sales that quarter. More details available in French on the Impots site.
This is the most straightforward of them all as you don't have to invoice for VAT. Add the wording "Exonération de TVA, article 44 directive 2006/112" to your invoice to make clear why you aren't charging it.
At the moment the UK comes under the EU rules. Depending on what happens with Brexit, those rules will remain in place for the transition period, but if there is no agreement, the UK will be treated as a non-EU company from 30th March 2019.