Travel from London and Bath to Fishguard and Newport

Bath to Cardiff > well, you could go by train & it takes about an hour. If you go by car, you head N out of Bath on A46 to join M4 heading west. (M = Motorway = like an interstate). However, if going by car, you may like to cross the estuary on the M48 bridge (free) & come off at first junction on Welsh side to then drive up A466 to see the ruin of Tintern Abbey. You could also divert into Chepstow to see the Castle - but zoom down to check out the one way traffic system on this map >https://www.google.co.uk/maps/@51.534403,-2.8153295,11z/data=!5m1!1e1

Please note that the M4 frequently jams up at Newport due to a 2 lane each way tunnel at a very busy section of motorway.

I would suggest that you would need 3 nights in Cardiff in order to see everything in the area: > Cardiff Castle, Castell Coch, Caerphilly Castle & St.Fagans Castle Museum and the National Museum of Wales (good for art) in the city centre. All of these can be done by public transport from the centre. If you have a car, it might be best to have a hotel that is not in the centre due to parking issues. If using public transport, a hotel in the city centre is the best option.

It is possible to go by train all the way from Cardiff to Fishguard but the trains are infrequent. A train goes every other hour from Cardiff to Haverfordwest and I would consider this if you are not driving - but check for car rental as it might be more difficult in the far west of Wales. Fishguard is mainly a ferry port for Ireland and tourists might prefer to stay in St. David’s. Newport (Pembrokeshire) is a nice enough little place. The main ‘resort’ in Pembrokeshire is Tenby - on the south coast and is well worth a visit. Further up the coast from Newport, New Quay & Aberaeron are both attractive coastal towns.

You are going in November and weather can be rough - especially on the west coast if Atlantic storms come in. Gardens will usually be dormant at this time of year. However, you may wish to visit the National Botanic Garden of Wales which is just E of Carmarthen - should you be driving from Cardiff to west Wales.
https://www.visitwales.com
Google the places mentioned.

If this is your first time driving left then in my opinion the hire car locations in Bath are going to be easier for you to drive and navigate out from than in Cardiff. That said I would consider a visit to either or both of these which are easily accessible from the motorway on the outskirts of Cardiff - Castell Coch and the Welsh Folk Museum at St Fagans. You can find overnight accommodation near both. That said, in November being outdoors (as you will be for 50% of the time at St Fagans) may not be the best time to see St Fagans. Might you be back for another trip in the future?

We stayed for in St. David’s for a couple nights and walked part of the Pembrokshire coastal trail. The cathedral there is interesting because it is built on a slope and the pillars toward the back of it, lean outward and towards the back. Near Newport, in Nevern, is St. Brynach’s church and the bleeding yew tree. Close to it, is the Pentre Ifan burial chamber. We also stayed in Tenby and liked it. Pembroke castle was nice to visit. We had a rental car and the main roads from Cardiff to Tenby or further west are an easy drive. Once off the main roads, there are some very, very, narrow roads at times, so don’t get a really large car. There were a couple roads where the bushes brushed against both sides of the car, and those were two way roads. Fortunately nobody was coming from the opposite direction. It’s all part of the experience.