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.

If you can drive from Port Isaac to Cardiff in 3 hours you are either breaking many speed limits or you are driving at 2AM - or you are the Google Maps directions generator!

Keeping legal in daytime traffic I would allow at least 4 hours and the only break in that is to pay the toll on the Severn Bridge.

You could look at the tiny village of Blue Anchor, perched on a cliff looking out to sea (for the sunset). It's off the direct route but about 3 hours from both ends. A nice walk is to Dunster, partly along the beach. I passed though briefly in June and put it on my target list for the future - so I can't vouch for any places yet but there are a few options. The route to it would involve the A39 Atlantic Coast highway - take a coffee break in Bideford - and through the Exmoor National Park

By the way, the fast roads in UK are called Motorways (with numbers having the prefix M). Expressways are something completely different and rarely involve high speeds.

Also most speed limit enforcement in britain is done by automatic cameras, either Gatso instant readings or "average speed". The ticket goes to the registered keeper of the car. If it's a rental, the company will pay the fine and then charge it to your card, probably with a service fee! I don't suppose Google Maps would pay it for you.....

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

The A30 is a "dual carriageway". For an example of what passes as an Expressway in England search for Aston Expressway on Google Maps and then switch to Streetview, then imagine what it looks like during rush hour! And yes, the red lane in the middle is usable and reverses direction to suit traffic needs!!!