Routes or Tracks or ..... ?

I will try your suggestion with the Open Source Maps but wouldn't that effect the routing much / or the re-calculation done by the device ?? My concern is that I want to keep the routes intact as MUCH as Possible and I don't want the GPS to create a completly different route.

The maps loaded into BaseCamp on your laptop don't need to match the maps in your device. I have several different map sets loaded into BC including the British Ordnance Survey 1/50,000 scale map of the UK which is from one of my hand-held Garmins. Now it's not the most up to date map (2013) compared with the latest City European Navigator maps (newly released this month) BUT the OS mapping has far better TOPO and other important detail than City Navigator maps, which are just sets of roads, grey shading for built up areas and POI's. Interesting stuff like ancient abbeys, Roman remains, Norman Castles etc. are simply not visible on City Navigator maps and only appear when you search the list of POI's. So I generally do all my route planning using the OS mapping even though it's routing information is way out of date.

When you send a route to the GPS device you are only sending the list of coordinates of via points/shaping points/waypoints etc. The device will then recalculate the route based on it's installed maps and routing algorithms. If you are using "Older European Maps" maps in BC now to do your initial planning then you are equally as likely to plan routes that may not resolve in the same way when imported into your new GPS device. So using Open Source Mapping now won't detract from your current planning exercise.

When you arrive in Europe and purchase a 770 (or whichever device you decide to go for) you will be able to connect it to your laptop and BC will automatically "see" the maps on the GPS device. You can then set BC to use that map set and recalculate each route to check if it still takes you where you want to go. If you register you GPS device online (and why not) using Garmin Express you will be able to download the latest and greatest City Navigator maps to your laptop anyway and also update your GPS devices maps at the same time (just make sure that you have a good WiFi signal as updates can take 90 minutes or longer).

I think that the key thing now is not to over plan either the global trip or each days route. By all means have an "A" List and "B" List of places you wish to visit, but you might well find that 2 months just simply isn't long enough!

One additional point to bear in mind is that the UK's exit from the European Union (Brexit) is scheduled for 29th March 2019. At the moment no one knows exactly how this will affect border crossings, vehicle warranties and insurance and travel into and out of Europe from the UK, which I guess in your case will be via either the Channel Tunnel or ferry crossings between the UK and France, Belgium, Holland or Spain. If you are hiring an RV then double check the T & C's as will be applicable post Brexit.

;)