Driving from London to Chepstow is really easy!
It’s one straight motorway, all the way.
Leave London on the M4 Westbound and drive straight to the Severn Bridge. This is outside the M25, but takes no more time than a tough evening on the North Circular.
For detailed directions, see driving to Chepstow.
When close to the Severn, just make sure that you take the old motorway, now the ”M48”, and the ”Old” Severn Bridge when crossing the Severn, not the M4 and the ”New” Severn Crossing. Once across the bridge, you’re at Chepstow. Leave the motorway on the first roundabout and follow the signs.