'Narf getting technical, innit!
Call Plymouth Mil for crossing if you go the Portland Bill direct to Channel Islands route (I would, for that nice cheap fuel).
Jersey - phone for PPR before takeoff (number's in the guides). I've never had a problem with them.
If you have IMC or IR, you can fly SVFR down to 3km in the CI Class A, othewise it's 10km.
Once you get to France, it's easy after you've cleared Customs. Make sure you land at a Customs airfield first, though. Once you've done that, you're free to roam!
Those French low-level mil routes aren't much of a problem if you do your plogging properly, and note all the frequencies so you can call to check/get transit. Most will let you transit if you talk to them; some won't. Be sure you know the French for your circuit calls and to understand the autoresponders that will answer you on those LF-R frequencies if they're unmanned.
Enjoy the trip - sounds great!