Pisa is the main international airport serving Tuscany, but you can also fly to Florence or, if you are visiting southern Tuscany, Rome. Flights from the UK to Pisa take just over two hours.
For getting to Florence by train, see seat61.com (note that you can no longer take an overnight train from Paris to Florence but have to change). Man in Seat 61 is also a good source of information on putting your car on the French motorrail from Paris to Nice (from where you can drive into Italy) or on the Urlaubs Express from Dusseldörf in Germany to Verona. Alternatively, it's about 13–14 hours' straight driving from the ferry/Eurotunnel port of Calais to Florence, costing an estimated £70 in tolls and £110 in petrol.