Let me add some suggestions here... :)
A personal favorite of mine (though I'm not alone) is the Bernina railway in Switzerland. You could do something like:
- UK - Paris - Strasbourg (overnight) -> expensive high-speed trains, must be booked ahead of time
- Strasbourg - Basel - Zurich - Chur or somewhere in Engadin (little hike maybe) -> very frequent trains, all reservation-free. Motto: go with the flow.
- Chur/Engadin - Tirano : the trip's highlight aka the Bernina Railway! In summer there are open-air wooden carriages, regular carriages with pull-down windows or the touristy Bernina Express.
- same day continue towards Lake Como and Milan: a replacement bus will be involved (upgrading works for the 2026 Olympics) but don't worry about it, it's well setup.
- Northern Italy - Trieste - Ljubljana: rather than Milan perhaps stay for a night or two in either Verona, Bergamo, etc.
- Ljubljana - Zurich or Stuttgart night train: very convenient and usually not too expensive. Book a private double sleeper ahead of time.
- Stuttgart - Paris - UK: you could head home straight after the night train. Do allow for a 1-2h delay into Stuttgart.