You've still got time for London - Paris but book well in advance for sure, especially if you need to take a specific train. Other trains won't be available to book until January - February.
TGVs between Paris and Zurich require a 29€ reservation (39€ in 1st class). They leave from Gare de Lyon so you need to take the RER (not included) in between. There is however a good alternative :
- 10 min walk from Gare du Nord
- TGV Paris-Est - Strasbourg 10€ limited fare, then 20€ (both classes)
- TER Strasbourg - Basel SBB (200 km/h)
- any train to Zurich
Yes availability a day or two before shouldn't be an issue in Italy : trains run at least hourly (up to 4 trains an hour between Florence and Rome). Book a bit earlier (or be flexible) trains around May holiday weekends, Ascension and Pentecost.
Thanks - I'm considering a 1st class pass - will this give better reservation availability do you think?
Yes definitely (even though there are less 1st class seats per train). You can always travel in 2nd class if there's no other alternative. Price is the same.
Eurostar is a bit more expensive (38€ vs 30€) but you get a small meal onboard.
Simply be aware that regional trains in Italy don't have 1st class carriages.
Last thing : you can easily cross the Switzerland-Italy border reservation-free. Take any train to Lugano and then the hourly RE service. Not longer than direct trains.
Btw if you've got time consider the Albula & Bernina railways to get into Italy. Stunning all year round ! More info : https://www.seat61.com/trains-and-routes/bernina-express.htm
No need to pay 25 CHF extra for the big windows, there are alternatives :
- reservation-free carriages at the front of the Bernina Express : less crowded, pull-down windows and in 1st class you can see the driver's cab. :)
- hourly regional trains on the same route : not longer but you have to change twice (easy and straightforward connections)
Questions welcome.
You can make the Italian reservations up to a few minutes before departure, there's usually plenty of availability.
The day before should beer absolutely fine, just keep an eye on it.