If the roads are showing similar delays when you set off, start on the M4. Near Maidenhead, have your passenger / siri / etc check again. If the M4 route is looking bad at that point, you can swing off onto the A404 up to High Wycombe then M40 from there without much time penalty.

Unless things go wrong, the A40 from Oxford is normally quite busy, but generally moves along. If you're not used to doing 50-60mph in heavy traffic in the dark on a somewhat windy and non-streetlit road, it might not be the best, but I've always found the drive fine (as a local)