There are so many more complexities to the plot of this wonderful thought provoking movie than just infidelity and cover-up of responsibility for the accident. I was struck by the initial seeming goodness of husband Wilkinson who wanted the driver, when he thought it was Everett, disclosed to the police, and the change of heart (and morals) when he learned it was his wife. As well, was he indeed good, and/or was he attempting to redeem self by allowing her to go with Rupert. Then, things switchedand SHE decided the right ting to do was admit that she hit him. Most importantly the theme of redemption (for the accident - for the infidelity - in her own odd, flawed way)is strongest in Watson's sickbed care for Everett. I believe that is why she undertook that effort.
This seems to be a common theme in modern British novels: Brideshead Revisited, The End of the Affair come to mind. Love it.