Hi, I think most sat navs that can be safely secured to your bike can do the job, it just depends on how much you want to spend. The more expensive ones can have features in, like showing weather, rainfall overlays (with the help of an app and suitable mobile phone) so that you can alter your route so that you don't get wet. Although you would probably have to divert via Spain at the moment to not get wet.
I use a free software package called "Tyre", which links into Google Maps, and you can plan your route via drag and drop, to go on the actual route you wish to take, and the sat nav follows that as a list of way points. It produces output files that can be used with TomTom and Garmin. Not sure if it supports others. Available here http://www.tyretotravel.com/
Enjoy John
