Just had to obtain an Emovis tag myself as France is in the process of removing cash and card payments at toll booths.
On the autoroutes that have the new system payments can only be made on the Internet or by having a tag or in person at some tobacconist, shops and service areas etc.
Emovis require you to purchase two tags
Liber-t for France and
Via-t for Spain and Portugal.
Other tag providers are available but Emovis is best for UK residents
as they direct debit in pounds sterling as opposed to euros.
Most UK banks don't allow direct debits in euros.
CAMC have an offer on Emovis tags if you are a member.