Went to Le Mans for the races last year, a good outing.
The route is straight forward. You might like to coast out some where around Shoreham if you are coming from Cranfield. Shoreham will give you a peace of mind service to the FIR or of course work London as you prefer. Make land fall around Deauville or a little to the West. Deauville will give you a good service and it is a pleasant customs stop if you feel like a break. Take care to avoid the danger area to the north east of Deauville.
From Deauville it is direct to Le Mans overhead Alencon if you wish BUT make sure you avoid the danger area almost due north of Alencon.
During the 25 hour race Le Mans is very busy and very expensive! You will need to book and have the AIC. Parking will be on the grass and their is a regular bus service especially arranged to take you to the tack (at a cost). It is about a 30 miniute walk otherwise. At that time of year it can get pretty warm so think about your aircraft's performance if you are near MTOW. The ground rises to the north of 02 and you can expect some turbulence over the rising ground in hot weather. There is customs during the 24 hour race but I think only by arrangement at other times.
Beware there is a danger area to the east of Le Mans if you decide to divert a little of track. Of course you will want to check the danger areas in case they have changed.
All in all a pleasant trip although the scenery en route is not particularly interesting.