Cross into Wales on the old Severn Bridge.
Turn off the M48 as soon as you enter Wales. Follow signs for Monmouth until you get to the Mitsubishi Garage then take the B4235 road to Usk. In Usk take the B4598 road for Abergavenny.
Stop for a cuppa at Abergavenny bus station.
Depending on where you want to go, pick a route accordingly.
Ride to Hay on Wye via Llanthony & Capel y ffin (take the Hereford road from town and turn off into Llanfihangel Crucorney for the minor road)
Ride to Builth Wells via A40 & then A479 - fast A roads with lovely scenery. Watch out for camera vans.
Alternatively, now that I've looked where the campsite is, cut across the Cotswolds to Gloucester. Ride via Newent & Fownhope to Hereford. Take the Brecon/Kington road out of the city. Head towards Kington then Rhayader. Turn off for Hundred House just after Radnor. Lovely fast A roads.
The A44 from Oxford all the way to Radnor is a lovely road too.
Once at Hundred House you are spoilt for choice for roads to ride. Just stick to the minor roads as much as possible.