I would take the train within Italy — fast, convenient and cheap.  I would fly from Rome back to London — way, way too long a train or bus ride and it will be expensive by train.  Look up pricing on www.eurolines.com and on www.sncf.com for your travel in France — I prefer the train and you might find some special pricing.

I would not buy a railpass for this trip unless a French railpass saves money on the trip between Paris and Nice and/or you’ll be taking advantage of the “passholder fare” (usually NOT a good deal for youth) on Eurostar, the chunnel train.

There are lots of reasons why train is preferable to flying, but I don’t think scenery is a major concern on this trip.  The trains run over 100 mph, so anything nearby is a blur, anyway.

I agree – use train travel within Italy and fly in and out of London. I haven’t used the train in France.