As you are coming in from Banbury may I suggest you enjoy the fine roads from there to the Welsh border. Chipping Norton, Stow on the Wold, Cheltenham, Newent, Ross-on-Wye.
From Ross, head west to my Home town of Abergavenny on the superb B4521.
From here, you are spoilt for choice.
Here's a favourite Sunday morning bimble: A465 up Black Rock to the highest point on the Heads of the Valeys Road. Turn off onto B4560 that runs from Ebbw Vale over Llangynidr Mountain, past Llangorse Lake to Talgarth. A438 from there to Gerry Huxtable's Bridge near Whitney on Wye. Back into Hay on Wye. Towards Ross on Wye until you meet the A465 (known as the Golden Valley and an absolutely superb road on a bike). A465 back into Abergavenny. About 80 miles.
Mapspource file below.
Allan has covered much of the area to the west, although he missed out the Aberystwyth Mountain Road from Rhayader to Cwmystwyth which is a favourite of mine. If you want some more roads in the border area to explore let me know.