I've ridden from Switzerland to Croatia many times.
When planning your route, I suggest you try to avoid riding on the motorway that goes across the top of Italy as much as possible. That motorway is boring as heck, the scenery around it is boring as heck, and the towns around the motorway are also boring as heck.
I usually do the west to east travel through Switzerland and the Western tip of Austria, then I drop down into the Dolomites
and the Julian Alps
and travel west to east until I am more or less above Slovenia. From there, I go south to Croatia.
The very northern part of Italy that comprises the Dolomites is superb riding, awesome twisties, great hotels and food, wonderful culture. Likewise the parts of Austria and Slovenia that you will pass through as you transition from the Dolomites to the Julian Alps.
Routing this way avoids almost all of Italy except the northern provinces that comprise the Dolomites, and the area around Trieste.
It is a bit longer route than taking the motorway, but the difference in the quality of the ride is like night and day.
From London, 4 days to get there and 3 to come back is kind of tight. You could do the route I propose on the 4 day trip outbound, but it would be difficult to do on the 3 day return. In fact, I would say it will be darn hard slogging to do Croatia to London in 3 days even if you take the least desirable motorway routes.