We will be in Giverny in September and will be flying to the UK from CDG. What will be the simplest way to get from Giverny to the airport without a car?
Take a the train from Vernon-Giverny to Gare St. Lazare (train station in Paris). From Gare St Lazare take a taxi to CDG.
You will need a taxi, or the shuttle bus, to get from Giverny to the train station in Vernon.
In other words, it's not very "simple." you are probably looking at:
1) taxi from Giverny to Vernon train station; then
2) train from Vernon to Paris Gare St Lazare; then
3) taxi from St Lazare to CDG (at least this last taxi is fixed-rate -- 53€ for up to four people and your luggage).