I suggest you post this on the London forum
The first question you might be asked is where and when are you planning to pick up the car ?
and you will then be told you are mad if you plan to drive anywhere in London !
Having done this journey hundreds of times, I always prefer to drive down to Cornwall on the M3, A303, and A30 route. It's actually quicker than the M4, M5 which is the route that most routeplanners would suggest but also much more pleasant.
The best thing about the M3, A303 and A30 is that you're not stuck on motorways for the whole journey and it is far more scenic. You even go right past Stonehenge.
You'll love Crackington Haven when you get to Cornwall too!
LOL Forgotmytoothbrush - You prefer the M3 & A303, we prefer the M4 & M5 !! Even though it is not picturesque, we find this quicker for us, even though it is a longer route. Good job we are all different or one route would be choc-a-bloc! But as caity asked for a scenic route, your route is more scenic, as I don't consider the Bristol Channel scenic.
As barbexpat said, your start point and time of leaving with really depend on where you might stay for the night. Unless you are starting late in the afternoon, you could easily make Crackington in one go with just a stop for food and the loo, but probably a more relaxed drive and a overnight stop is part of your holiday?
There are some pretty Somerset villages, and towns you could spend the night in, then you would only have a shorter drive the next day. We stayed at a B&B just south of Shepton Mallet last weekend.
Thank you all for your replies. As yet are still organising car hire. Our original plan was to catch a train from London but it is much cheaper though not more convenient to travel by car from London as we are arriving on a Saturday. We arrive in london at 10.00am so looks like we could travel the whole distance in one day. The girls are quite excited about going via stonehenge so a stop over is still on the cards. To us the journey is as important as the destination.
It probably wouldn't take any more time than driving and once at Heathrow you are right on the motorway and it is easy to go whichever route you prefer from there. It would also probably be a lot less stressful.
at that time of year I'd also suggest the M3 route. Depending on your budget you could break your journey somewhere like Exeter (pretty town with walls, cathedral, river and good shops). You could drive down in one day but it would be tiring and a bit of a shame not to see anything on the way down.
What is the time difference between taking the M3/A303 route vs the M4/M5 route? Are we talking more than an hour more or less?
A303 timings are more dependant on traffic as there are some sections of single carriageway where you can get stuck behind a lorry, tractor or tank (Lots of army about!).
The all motorway route is more predicatble, but I nearly always use the A303 as it is pleasanter. Although the driving is slower the distance is less so it shouldn't normally take more than 1 hour longer than the motorway route.
I would agree with the M3, A303 etc, route if you are driving. Much more intersting. A bit slower roads but more scenic on parts. The M4, M5 if quickest but I find it bit monotinous for most parts.
www.theaa.com has links to maps & can search route.
Click "Route Planner" on left hand list. You can then add "via" route & put M3, A303. Also shows map. You car may also have Sat Nav toy enter this route.
The great road debate also really depends on your London starting point, we are off to Surrey then Gatwick next week and will go A30 A303 M3 as it is south of London. If we go towards Heathrow, we go M5 M4 M25.
I the preference for M5 M4 comes because we also do Oxfordshire quite a bit, which on a good run (ie picking our times to travel) we can do Penzance to Oxford in 4 1/2 hours. We probably have a really good run as everybody else is on the A303 in traffic jams (only kidding please be kind traffic God next week)