As many have said before, the further south you go, generally the nicer (and hotter!) weather. We are regulars to Carnac - generally get 22-25 degrees in early August. The region is great and can highly recommend it - towing our old outfit there was never a problem.
The route and options are down to personal preferences - we've done the ferry to Caen and Cherbourg a few times but we now much prefer taking the Eurotunnel to Calais. Timing-wise to our destinations they are generally similar, but the tunnel is by far cheaper (especially using Tesco vouchers!) and departure/arrival times work better with when we want to get places.
We went to Argeles-sur-Mer (near Perpignan) last year (without the caravan) and broke the journey up over 2 days (may have needed 3 towing or 2 very long days). The weather was much hotter, but we did go during the heatwave last year so regularly saw 38-40
hmy: . But the beaches and warm water were much nicer than the north!!
If we get another outfit before next summer we may look at the Vendee region as a compromise between travel time and seeking warmer weather!