We live nearer to Poole than you do Chapmag but do find getting through Poole an absolute nightmare. It took us over 3½ hours to get from Salisbury to the ferry terminal in August, in fact we did not arrive until a quarter of an hour before the sailing. Traffic was awful.
We rang Brittany Ferries booking office to tell them we were stuck in traffic and would not make the check-in times and they said that we would have to go the next day. However, they gave us the number of the ferry port at Poole who, when we rang, asked where we were and said not to worry even though we would be past the check in time. We thought we had left it a bit late but when on board we watched as a car and caravan came on board a minute before the due sailing time. The Dorset Steam Fair and road works were the cause of the chaos.
I notice that the timetable has changed for next year and the 12.30pm sailing is no longer available. Pity about that as it used to get into Cherbourg at 6pm, allowing plenty of time to get to a site for the night.
Thanks for the response.. and the info. about the 12:30pm sailing.. that's the one we would normally use.. I hadn't spotted it had been moved.
This year we are going to France for 9 weeks starting 5th May and as the Eurocamp Independent booking was almost the same price for the Portsmouth -> St Malo sailing as the Poole -> Cherbourg (without the fuel and site fees to Swanage/Poole) we plumped for that route this year.
I have to say that we do play it safe when using Poole and travel to the CC site at Swanage the afternoon before and then drive the 16 miles to Poole mid-morning on the day of departure.
Never had a trafiic problem in the last 5 or 6 years but we did go outbound early (ie late May early June).
Back to the OP's journey (if using Poole) I'd recommend they come down the night before and stay on the dockside for the early ferry.
That way any problem with traffic is negated and they get a good run to St Malo (maybe 3 hours) in the afternoon/evening.