I'm one of those strange beasts, a CAA/PPL with SE-IR, but I've not got any FAA paperwork, so will have to refer you to others for what you can/can't do with that.
A number of questions posed, and I'll try to answer some of them now, and have a go at some more later ;-)
IFR in France:
A flight plan is mandatory, and depending on how you file it will dictate how much is on/off airways. If you file in France using the BRIA over the telephone, you will get away with a lot more than if you file by fax, or from the UK (when CFMU kicks in, but thats another story!) If you file an airways flight plan, at typical non-turbo heights, when airborne you will frequently get a direct to a waypoint on your route that avoids some of the doglegs. Sometimes the airways are not suitable routing/available FL's, and if not filing over the phone, you will have to accomodate the 'max 50nm between DCT waypoint' limitations, which can be challenging itself
The Ground Studies:
I did the correspondence course with the now defunct PPSC, and despite all the rumours, the amount of irrelevant coursework I though was fairly insignificant (but then maybe one day I will have an a/c and the do$h to get weather radar, or I might decide to do a long trip and use HF kit) For me it was a mandatory 1 week on site course, and I learnt wads - very definately worthwhile. I guess something like 40% of the course is Met, which personally I feel that no IFR pilot can understand too much about. The exams were one and a half days at Gatwick, (most of the exams have a generous time limit), but the training made that a one off exercise. At the end of the day, the ground studies were the cheapest part of the process, with the exception of getting the audiogram added to the medical (half an hour or so with a pair of headphones on, waving at the AME at suitable intervals)
The Flight training:
The goalposts have moved again since I did mine, but I believe there may be an reduction in training you can claim if you already have an IMC. Of the whole course, a certain amount may be completed in a sim, and I would heartily recommend you spend some time in one (I really got to hate it, but as a learning/teaching aid it was superb device) I had a slight conflict with my instructor when I first started, but as soon as he realised I was doing an IR to fly a puddle jumper round Europe, and had no interest in maintaining a CDA to LHR in a 747, we worked it out.
The Flight Test:
2D's wrote an excellent article in one of the recent Flyer mags which compared many of the aspects of the CAA vs FAA flight tests and methods, but the CAA test is very much a 'cockpit workload management'. My initial test route was EGHI-NEDUL-EGHH for a hold and couple of approaches, ADSON (and the surrounding area) for the Limited Panel and UA's, and then a recovery to EGHI. The problem with this is the first leg, with climb, is still less than 10 minutes long, and in that time you have to fly the a/c, comply with the SID, complete all the checks (including SIT'ing all the navaids), negotiate the arrival and requirements at EGHH, and get the Bournemouth weather in, all before turning inbound to the BIA. You then get a whole 6 minutes or so to complete the approach brief, plan the hold entry and any WCA's you want to use, perform a fuel plan check and so on. It's a busy period!
Using the Rating:
Annual signoff, and unless you let it lapse for ?5 years or more, the process is the same. The last time I did it, a friendly CRE from my local club (White Waltham) jumped in with me into my a/c, and he subsequently signed off my IR (for a year), IMC (25 months) and CofE (2 years), and charged me £65 (fairly sure it was that much, getting on for a year ago now). Because its single engine stuff, I'm not desperately intersted in 250' minima (but it does help keep the flying sharp to do it), for me it means that I can get into N866 over to Jersey
, get above some types of clag in France/Switzerland/Germany, or even fly through it. It also means that the route between Bristol & Edinburgh no longer has an enforced descent around Liverpool, and generally opens up more routes. Flying airways is actually very easy, constant Radar control, probably one squawk for the trip, handovers between each ATSU, and more than likely the cruise is all on one altimter setting. On the downside, getting flight plans into the system can sometime be a pain, occasionally (but rarely) slot times may be issued which you don't really want to miss, and probably the biggest: making the decision if you want to go when the weather is a bit 'iffy' - whilst you could probably go quite legally, into something that you would otherwise not consider.
Fully agree with the previous suggestion that you join up with PPL/IR, hope this all helps
IFR