Real shame to hear of this loss. Heart felt condolences to family and friends.
I am not familiar with the section of road but I would have to ask was this after a bend or similar so that the driver could not see far enough up the road. If you come round a bend, get blinded by a low sun then this could happen. Not sure it should happen on a clear road though as you will get moments of vision so that you can see what is ahead.
If you are driving along at say 50mph and you become blinded by the sun you do not do an emergency stop, it would not be safe to do so. I would expect you to slow down as you try to get clear sight of what is in the road in front of you. You should have a good idea as to what was in front of you and roughly when you would expect to reach it before you were dazzled.
Given that this is the A40 at rush hour I would kind of expect there to be a fairly steady stream of traffic. Further up the traffic I would expect to see cars braking and then pulling out to pass the cyclist. This should have been sufficient warning I would have thought. But how many are looking that far up the road when driving??