Since this is a European train forum I might suggest to do the whole journey by train from the UK!
There are beautiful Alpine railways and it would be a shame to miss them. For example the Bernina Railway fits perfectly on a UK-Italy itinerary. A relaxing way to cross the mountains after high-speed trains through the French countryside. See:
https://www.seat61.com/trains-and-routes/bernina-express.htm
https://www.seat61.com/trains-and-routes/bernina-express.htm#Bernina_Express_to_go_from_London,_Paris_or_Zurich_to_Italy (incredible website btw)
Also possible to take a direct Paris - Milan TGV but the views aren’t as gorgeous in my opinion. The route is closed this year due to a landslide but should have reopened in 2025.
I’d make my way slowly south, especially as the weather will be better up north at the start of the month. The Tuscany region is really cool and there are plenty of day trip opportunities.
At some point you’ll have to cross the Messina Strait, do it using the last European train-ferry!
https://www.seat61.com/trains-and-routes/trains-to-sicily.htm
For the way back you could take the convenient night train to Milan (from either Siracusa or Palermo). Of course you could fly back from Sicily if you prefer.
These are some guidelines which shouldn’t change much until 2025.