That is one hell of a long journey for you. Bristol - Dover - Le Mans. Surely there is a quicker if not cheaper way.
Agree about the use of Speedferries. One hold up on the way and you're in for a long wait. If you miss your P&O or Sea France they put you on the next sailing, 40 minute wait is not so bad.
A friend of mine uses the Portsmouth Carn route but its up to 10 X the cost (le mans week) of Dover Calais. As piglet says I used Portsmouth Le Harve for the moto 24 because I could, sleep in both directions and go out later then be back for work Tuesday.It was a cheap ticket and worked fine, I tried to book for Le Mans all of the overnight ferries were 400-500 quid for that week.
The SeaFrance ticket we get fully refundable and amendable tickets, so we just turn up and they get us on the next ferry. 4 hours to Dover, 90 mins on ferry and then 5 hours to LM via Calais,or 2.5 hours to Portsmouth, 7 hours on ferry, and then 3 hours to Le Mans from Le Harve I know Caen is a little closer but you only save an hour
It's about 1000 miles of driving ( LM and back via Dover) so about 20 gallons of diesel, lets say £80 for fuel and £100 for the ferry return against about 500 miles to LM via Portsmouth so £50 of fuel and up to £450 on ferry return so about £500 against £180