We've driven to Tuscany from Glasgow, taking our time and enjoyed the trip. We are however, seasoned long-distance drivers - have regularly driven to Meribel for example.
Here are some thoughts from our experience
If you're going to cross via the channel ferries, pay the extra for the Club Class (P&O): the extra £10-15 per person really is worth it for the haven of peace (and the glass of bubby for the adults, plus unlimited tea, coffee and biscuits) means that you start the new section of driving refreshed and relaxed. Do it with the prioirity boarding (which gives you a discount for the Club Class) and it means you are ahead of the traffic not in the middle of it.
Driving on the right: is really not a big deal and it is amazing how quickly you get used to it. We find traffic on the continental motorways to be much more disciplined than the British motorways. Once you have developed your confidence on the motorways, you could consider going on the N routes which tend to run parallel with the motorways and give you a better flavour of the countryside - and the opportunity to stop for breaks in "real" French towns.
Ignore traffic routes that take you anywhere near Paris. From Dover, you take the A26 then the A6 - and then you have a range of otpions further south.
When doing your sums, you need to factor in the cost of the peages (French toll motorways) and the equivalent in Italy (can't remember what they're called)
Consider getting the ferry from Hull to Zeebrugge. I think the Rosyth ferry has stopped - but the Hull ferry gives a different bit of "holiday". It's not so much the drive down to Dover that is a pain - it is the drive back up, after your holiday, that is a pain. And Hull is that wee bit closer.
If you do go via Zeebrugge, plan to fill up in Luxemburg - it is much cheaper there.
If you are going via France, don't assume it will be cheaper there. Do your research on the internet before you go - last year, it was cheapest to fill up at the BP garage just before your arrive at the ferry docks in Dover (and FWIW: the cheapest petrol on the way back up first filling station you come to on the M74, ie the Scottish side of the border).
Make the journey part of the holiday: we stopped overnight in Fixin in Burgundy and found a fabulous wee hotel where we had an unforgettable gourmet meal.
Burgundy is one of the bits where it is definitely worth coming off the motorway and driving down the N1 through the french wine villages between Dijon and Beaune (we always stop in Nuits St George for breakfast).
Take plenty of provisions for the car - but also be prepared to stop regularly to stretch your legs. Will you be sharing the driving?