I will make another suggestion: I see that you have asked about staying in Premier Inn’s. There is a Premier Inn on the east side of Newport - just off the M4 at Langstone = the bit that keeps jamming up.http://www.premierinn.com/gb/en/hotels/wales/gwent/newport/newport-wales-m4-j24.html
Now, it would be possible for you to come up from Cornwall, leave the M5 for the M49 after crossing the Avonmouth bridge (west side of Bristol), then over the estuary on the M4 bridge with this Premier Inn being only about 15 minutes from the bridge. So, this location would be quicker and easier for you to reach than going to Chippenham and it has free parking outside. Now, this location could be used for you to visit Cardiff and Bath. Bath is around a 45 minute drive or you could drive to nearby Severn Tunnel Junction, park the car and catch a train into Bath. (If driving, leave M4 for A46 and I would suggest you follow the signs for the Park & Ride = where you park the car and take a shuttle bus into Bath - when (if) you sit on the bus, you will soon realise why this is sound advice.
The Premier Inn at Langstone is also well placed for the beautiful Wye Valley with Chepstow Castle being about 20 minutes to the east and Tintern Abbey just a short distance up the A466 to the north.
The A449 expressway goes NNE from Langstone - take this to Raglan and then A40 via Crickhowell into the Brecon Beacons National Park. (See Raglan Castle on return - should you do this). Go down a coal mine (free) at Big Pit , Blaenavon, is also easily doable. Newport, itself, is a dump = avoid.
http://cadw.gov.wales/daysout/tinternabbey/?lang=en
http://www.breconbeacons.org
http://www.visitmonmouthshire.com/?AskRedirect=true