Probably the best way to get from south Oxfordshire into London (carry on to the A404/404M to join the M4 [J8]). Very pleasant, crossing the Thames a couple of times, good opportunities to overtake, nicely wooded, and not long to west London once you're on the 404.
Fast A roads from Didcot to the A404 near Maidenhead with the only sticking point is the traffic in Henley, though thats mainly through commuting time.
If you're going the opposit way there's sometimes a speed camera van at the top of the dual carriage way out of Henley. It's national speed limit droppin to a 40 for about 1/4 of a mile.