As per the Garmin website:
Favorites and Saved places are organized by proximity only, meaning the Favorites and Saved places that are closest to your current position will display at the top of the list. Currently, this is the only way to organize Favorites/Saved.
You say that the favourites you have created are miles away in France and Germany, the closest 250 mikes away at the Channel coast. This maybe explains why they do not appear. I have lots of favourites on my device, each of which display in proximity (to my current location) order, the nearest or first on the list is a friend’s house in London, then another friend’s, then a petrol station on the M20, then the Chunnel and so on. You do not tell us what other (maybe closer) favourites you have on your device, which might be shown when you summon up ‘Favourites’.
I have no idea what additional complications or failures may come about by using MyRoute’s software to create routes / favourites. However, if the points are shown as ‘Where to’ destinations when you activate the route, it is safe to assume that the device knows that they are there.
PS It is generally bad practice to create one single, presumably very long, route. If I read you correctly, you have created one long route, connected by no less than 14 waypoints (points that you must go through) from your house to the Chunnel (250 miles) and then onwards for presumably many more miles through France and into Germany, a total distance of unknown (as you haven’t told us) miles. Are these 14 waypoints / favourites:
A. Single overnight stops?
B. Points that are nothing more than shaping points? In other words, points that you have created in order to do nothing more than to force your MyRoute created route to take a certain direction?
If they are A, try breaking the route up into distinct day by day chunks, each chunk being one day’s ride. After all, you are (presumably) not going to ride the route home to Germany in one go? There again, maybe you are?
If they are B, then they do not need to be favourites at all. They can be converted into unannounced shaping points, quite safely
In short, how in MyRoute, did you create your single route? Did you say to the software, “Take me from home to the Chunnel, to town A, to town B, to town C, to town D, to town E” where each town is an overnight stop, and then let MyRoute fill in the gaps in between? Or is there something more significant about points A, B, C, D and E? Have you put them in simply to force the software to take a specific direction in your (apparently) single route between your start point (Home) and the Chunnel in Kent and your end point, somewhere in Germany?