Port Isaac to Cardiff

It's not a village, but I suggest Wells, Somerset.
Cathedral and associated buildings par excellence.

It's only a three hour drive, so really no need to spend the night. If you are not already planning on spending the night in Port Isaac, why not spend the night there then the next morning to Cardiff. Topsham is about half way, it would be a nice stop, if you really want to stop for the night.

If you have a clear run - it is just over 3 hours driving time - mainly on expressways. Here is the fastest route:> http://www.theaa.com/route-planner/index.jsp#fromNode=0%7CPort%20Isaac%20PL29,%20UK%7C%7C-4.829978%7C50.593820%7CtoNode=0%7CCardiff%20CF10%203XQ,%20UK%7C%7C-3.178864%7C51.486627

However, I would not follow that route outtake a longer more winding route via the Exmoor National Park. This involves following the A39 all the way to Bridgewater - where you join the aforementioned fast route (M5). Using the A39 route, I would make first stop at CLOVELLY. Then continue east passing Bideford & Barnstaple - now it really gets bendy. Next place where I would stop is LYNMOUTH. Then head for Porlock and maybe have a quick peep at ALLERFORD = a tiny village on the left. Roughly mid way between Porlock & Minehead - turn L up a country land for a short distance to SELWORTHY = all thatched cottages. If time drive into Minehead maybe - but definately stop at DUNSTER = just E of Minehead. Google the places mentioned for more info. Dunster to Cardiff takes about 1 hour 45 minutes given a clear run.
http://www.exmoor-nationalpark.gov.uk
Please note that the above suggested route via the A39 is on some bendy roads with steep hills & is about 4 hours 40 minutes driving time. You might like to consider staying somewhere near Minehead if using this route.

I don’t think that Wells is worth the hassle of coming off - and back on the M5 (interstate) via some bendy ordinary roads that can be busy. Topsham has very narrow streets and is also quite ‘fiddly’ to reach even though it is right be the M5.

Thanks for all the help! We're mainly looking for small, walkable pretty villages on this trip, so you've given me some great ideas. We are spending a couple of nights in Port Isaac (big Doc Martin fan and just want to see that harbor) and will hit some of the villages in day trips.

The A30 from Exeter into Cornwall is an expressway and has a 70mph speed limit over much of the route.