Being from Rome... I've done it a few times with the motorbike (and a lot of times by car).
I've gone through Switzerland the most: London - Nancy, Nancy - Milan (going through a few passes around the Susten) then Milan to Rome, going through Val D'Orcia in Tuscany, basically riding the Cassia all the way down to Rome.
I've done the Route Napoleon variation a few years back, when riding m GS back to London from my mechanic in Italy, with a mate in March (too early to go go through Switzerland) and it's amazing too.
In that case we left Rome, via Cassia till around Florence, then took the FI-PI-LI towards the sea and proceeded to reach a B&B near the border with France.
Day after we had breakfast in Montecarlo a then Napoleon till Dijon. Last day is the boring bit to Calais, even though this September (I went to Ventimiglia for a couple of days) I found a nicer B road variation that eliminates a long stretch of motorway (if you wanna get to Rome in 3 days, there is a fair bit of motorways included here and there to speed up some bits).
Last but not least, I've rode to Rome for just a weekend once (had to attend to a wedding).
To speed up things, I've experimented using the train as well, so: London/Paris-Nice/Rome and then way back through Switzerland as above. That's 2 days going down, 2 days in Rome, and 3 days traveling back.
This is the way back
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I should have some (messy) GPS routes on my computer.