If it is the Sunday then options to split at Didcot Parkway are more limited due to engineering works between Chippenham and Swindon. It can still be done by travelling from/to Bristol Parkway, but the Bristol Parkway departures calling at Didcot outbound AM are at 0949 or 1102. Nothing earlier. Returning from Paddington there are departures to Bristol Parkway that can at call at Didcot at
The original poster was looking for arrival times by 10am, so even the earlier of these would not be suitable.
As it happened I've been looking at times and fares for the Sunday too, as we're off to Wimbledon. The prospect of a nice scenic journey along the
B&H▸ only adds to the appeal of the day.