I don’t know how long you are staying in the Trecastle area but I would like to mention some places in this part of south Wales. Please click for a map:>https://www.google.co.uk/maps/@51.919305,-3.7266731,12.07z
Right, now I guess you can see Trecastle. Look to the south of the map and you will see The National Showcaves of Wales - well worth a look - you could take the backroad over the mountains. Having done the caves, you may like to continue south for about 3 miles and then hard left and head for the upper Vale of Neath (Glyn Neath) and turn left as traffic lights and go to Pontneddfechan. Park here and walk up river to Scwd Gwladys/ The Lady Waterfall = takes about 35 minutes.
Look at the south-western part of the map - can you see Carreg Cennen Castle = spectacular ruined castle on top of a crag - with a passage that leads into a cave under the castle. You could try and drive on the backroad form Trecastle via Twyn Llanannan. However, it is easy to get lost on the narrow lanes through Gwynfe - where you may have to drive backwards to a passing place. The road up from Llandeilo is much better. Just west of Llandeilo is Newton House (NT) and Dryslwlyn Castle (ruin) and Aberglasney Gardens (not NT). The National Botanic Garden of Wales is also not far away from these.
Can you see Llandovery? Going north from this town, a backroad leads up to the Brianne dam. The road continues through remote country northward and you could then take the old drovers road back down to Abergwesyn and Llwrtryd Wells - don’t do it if you don’t like single track roads with passing places - check out on street view.
So, you are looking for places in England while drifting back towards Heathrow and you have seen a fair bit of Wiltshire. If you have not been to the lower Wye Valley - that would be a good option - still in Wales. So - A40 from Trecastle to Abergavenny and on to Raglan - visit castle. Then head for the lower Wye Valley - probably going to Monmouth on A40 and then A466 going south. Visit TINTERN ABBEY (ruin). As an alternative - A40 skirting Monmouth & Ross > M50 and come off at Tewkesbury. Then head east to Toddington and the northern Cotswolds (Broadway, Chipping Campden/ Hidcote Manor Gardens NT, Lower Slaughter, Snowshill, Bourton-on-the-Water).
Another alternative is to go from Trecastle into the more populated Cardiff area. Dyffyrn Gardens - just west of the city is NT but if doing this, I would first visit Castell Coch, then St.Fagans Castle & Museum of Welsh Life (takes half a day) & then Dyffryn Gardens. Also in this area is Cardiff Castle (in centre of city) and Caerphilly Castle - 7 miles N of Cardiff.
Google places mentioned if interested.