Cdr. Clouston's oldest son, Dane Clouston, wrote to the filmmakers to protest the decision to depict the Royal Navy piermaster as a "composite" character. Producer Emma Thomas replied that all the characters in Dunkirk were fictionalized "out of respect to the real-life heroes." 

And also, to avoid more accusations that the film was "historically inaccurate," or missed someone.

Historical "real life" accuracy--be it timeline, characters, events--often doesn't play well as gripping entertainment. It just doesn't. That's why many non-fiction writers turn, eventually, to novels--to fiction--a writer often can "capture the moment" (or moments) much, much more satisfyingly in fiction than in "real-life" non-fiction. 

It would have been nice, perhaps helpful, certainly even prudent, if producers had contacted the Clouston family as the script was being written. That way, the family perhaps would feel "included" and this kind of acrimony might have been avoided.