Hi Chris
It's about 110 miles from the port at Caen to MB, I can do it on a tank with the Duke so you should have no problem with the Kawa if you splash at Portsmouth.
As has been said elsewhere if you are coming down from Cherbourg there is a gas station signposted off the Farlaise by pass, at about 100 miles, but it's attached to a supermarket so the usual times of operation will be if force for cash or UK credit cards.
Would recommend Portsmouth/Caen especially if you are returning on the Sunday, however to my knowlege there is not a petrol station open for cash/UK cards on this route on a Sunday, so a full tank when you leave LM is a must as the next guarenteed fuel will be at Portsmouth, I've even had problems finding a non French credit card only fuel stop on Monday's at lunch time.
If you fuel on Saturday you can use a lot of fuel just running about at LM over the weekend, which can leave you short to get to the port, but I expect you will prefer to have a 'few' beers rather than be zooming about on race days, or prehaps Ian can take down a spare tin for you to top up.
If you are having a leisurely trip back on Monday I can give you a good bike route back to Caen.
Checked brittany fares about 2 weeks ago on Portsmouth/Caen, was quoted £125 for me and bike, overnight Tuesday ( with reclining seat ), back Monday afternoon, not bad as if you booked 2 weeks before the race this year it was £225.
Cheers