Where are you based in the UK and so which channel crossing do you intend to make? Likewise, do you have a particular point in mind on the Med as cycling to say Perpignan will be very different to say cycling to Nice?
Are you looking for an easyish ride or tougher more scenic route?
In any case, make sure you stay off main roads - Michelin yellow series maps are best for planning scenic routes and identifying scenic towns and landmarks along the way, you could do worse than this by way of choice of maps, or at least buying the Michelin road atlas to plan ahead to help decide your route.