My parents live near the beginning of the M4 so I'm often driving from here to there. On balance, that is still the best route, but I'd say avoid the earls court trap. So at the end of chelsea embankment cut through lots road, on to new kings road, straight past the bottom of wandsworth bridge rd, right at parsons green, up to lillie rd, fulham palace rd, hammersmith roundabout, A4. Or better yet, take the train from paddington, only 2 hrs to cardiff...