French postal rates go up on 1st January 2014.
The cost of sending a "lettre verte" will rise from 0.58€ to 0.61€ and the "lettre prioritaire" rate will rise from 0.63€ to 0.66€.
We shall be able to make a small saving by buying un-denominated (valeur permanente) stamps in advance. Note, however, that only those marked "Europe" or "monde" may be used to send outside France (letters only); other stamps without values printed on them (lettre prioritaire, tarif vert) may not be used for international letters/packages.
I'm sure I've stuck a couple of Mariane rouge on stuff to UK if I was lacking a blue one and I'm pretty sure the letters arrived. TBH I never noticed the "Europe" on the blue ones till I read this post.