Note that, for planning purposes, Memory Map do France as well, in fact their IGN of all France is something of a bargain, as map software goes. Though Google Maps is also very good for planning.
For your London-Paris project, possibly the best bet (without embarking on a steep learning curve) will be to hope some helpful person here will make you a OSM file to cover the requisite area, that you can then copy to your SD card. OSM is currently rather incomplete in Northern France, so you may be lucky with your precise roads, or you may not. If not - bring back a track!