Hi Bosbeer,
as my Mrs and I live in Italy we have done this trip a few times. We unfortunately always seem to be in a rush and we go from Modena to Blackpool in around 26 hours ish. This is WAY too quick to enjoy the trip and you could very easily spend a week each way stopping at different places along the route.
If you can stay off the French motorways, do so, if only to avoid the annoying, one-every-5-mins motorway tolls.
We have been Portsmouth to Le Havre (£80ish but free sleep) but usually Dover Calais (£50-£60 return).
We now drive through Switzerland just because it is so beautiful, the only downer being that we look out of the window and pine because we don't have time to stop (An annual motorway pass is around €30 so this might work out a bit expensive for a holiday. As we do the trip several times a year the cost is relatively low)
GBMUD is right about the driving in Italy. Just treat every road in every direction on a roundabout as a give way and you'll be fine. After a while you will probably join the using every lane and not indicating club, otherwise you just look and feel really foreign.
You will have a great trip, let us know how you get on!