I've been Pompey-Cherbourg for the last 12 years - I live in Somerset so the shorter crossings are not really an option. As JP says above, it takes 3-4 hours, about 170 miles to the circuit. Good road until Falaise, then typical French 'N' road until just before Alencon, then motorway from there. The last few years we have got off the 'vomit comet' at about 6.00pm, getting to LM at about 10.00 after stopping to get basic supplies (beer & wine), and that is in a mundane car dragging a trailer. Still enough light at that time to set up camp and sample the basic supplies.
Because of the P&O situation now, we have changed to Pompey-Caen. Hope this helps.
DB