I had just a problem in the South of France. I took the Carrier off automatic and clicked 'Other' and eventually all the carriers came up and I selected Orange which is usually the one I find comes up in my area. I left if there for the rest of my holiday. Bouygues was useless where I was and frequently said No Service at the top of the screen.
I spend a lot of time in France as we have a place there. I have to say that unfortuantely what you have experinced is farly common with O2. my wife has an EE 'phone and never has these problems.
Bouygues always seems to be the defualt partner for O2 in France a manual serch always solves the problem(as you have now discovered). Dont expect any 4G either as O2 has no 4G roaming agreement in place in France. The best you will get, and this will only be in towns and cities is 3G+(H+) .
At least O2 doesn't seem to be throttling back on data speeds any more. Not tried in France yet as we are not due to go over until later in Oct but I was in Belgium a few weekends ago and it was much faster.
Track usage, upgrade and get support with the My O2 appDownload My O2