Sean,
The first comment I have is that if you're planning to stay in Lyon for 3.5 months and travel in Europe for an additional month, have you considered the terms of the Schengen Visa? You would clearly be in violation of the three month maximum limit for tourists, and could face severe penalties. However, if you have a student visa or whatever, it shouldn't be a problem.
I used a SIM from O2 network on my trip this year, and it worked well. However, I obtained the SIM from one of the "travel SIM" firms here, so the rates are different than those for U.K. customers. The SIM I was using provides a consistent rate in most western European countries, with NO "roaming charges".
You will likely only be able to get a PAYG plan if you go with one of the U.K. networks. You might check the websites for some of the carriers to get some idea on what your costs might be, considering the fact that you'll primarily be using the phone in France and other countries (and therefore roaming). Some that you might consider are O2, Orange and Vodaphone.
You might also have a look at the "travel SIM's" and compare costs to see if that might be a better option (again, since you'll mostly be roaming). Have a look at Roam Simple, Call In Europe, Cellular Abroad, Telestial or Mobal for details. The cost for these is charged to a credit card, so never any worries about "topping up". It's a darn nuisance to run out of minutes in the middle of a call!
You could also perhaps buy a SIM in France when you arrive, however again you'd be roaming when in the other countries. If you buy a SIM in Europe, be sure to confirm the "topping up" procedure.
Be sure to check your "unlocked phone" to ensure that it does in fact have the two European frequencies (that's not always the case). Also, check the Charger to determine if it's designed for "world operation".
Happy travels!
