Are you travelling alone? If not, maybe a good idea that the passenger has a variety of toys/distractions to hand. Keep all the essentials to hand, not buried in the boot i.e. changing bag, spare outfit, bottles if applicable, etc. If you're alone and baby is in the back, do you have one of those backseat mirrors so you can see them? Quite cheap in Boots and Mothercare etc.
I drove to Scotland from London when DD was 5 mths, and to be honest, she slept most of the way. It took nine hours door to door. My best advice would be to start early and accept that these things take longer with a baby, and whenever you stop the get the pram out and let the baby have a good long stretch not in the car seat.
I would say that I find DD gets hot in the carseat even if it's cool in the car, so don't bundle baby up with jumpers/coats etc. Far better to dress them in something comfortable and cotton, without potentially restricting waistbands (so a babygrow or similar), and put a blanket on top of them if necessary.
Ooh, and don't forget to make sure they are out of the sun, so sunshades at the ready if it's a nice day.
Take snacks for yourself. And drink tea.
Bet baby will sleep the whole way... Good luck!