Wi-fi is widely available in France for free at every McDonalds (where there is often mains available inside) and Pat a Pain. Unlike in the UK, at McD's in France you don't need to register to use the 'service,' just click on agree and away you go. Many municipal sites also provide site-wide wi-fi which is free (if the site is expensive) or quite cheap - at Chinon it is €1/day or €5/week for unlimited (and fast) use.
Rather than use Skype which has been having problems across Europe lately (and which can often be blocked and may get much worse now they are owned by Micro$oft) open a VoIP account in the UK (I use voip.co.uk) and download a softfone onto your laptop (Phonerlite is very good.) Then once on line, open the softfone, see it register, and call home. (You can get USB handsets for less than a tenner which make life very easy if you don't want to use internal speaker/mic.) I pay 1p/min to call a UK landline, the speech quality is very good albeit sometimes with delay, and your CLI is a UK number (as it is for someone to call you.)
You don't want a French SIM. They cost about €15 plus €10 minimum loading, the system only recognises putting more money on the card as 'usage,' and if you don't make calls it for a particular period - usually 8 weeks - the card ceases and you loose any credit. Most of them you cannot add credit outside France as they use shortform numbers for the purpose.