Why are the English referred to "dogs" as an insult?

● "Why are the English referred to "dogs" as an insult?"

I think you missed out the word "as" - needed to make it «referred to as "dogs"»
The answer is going to depend on WHO uses it that way.

💥1: English composer Noel Coward wrote https://www.youtube.com/watch?v=vMlyT_Sb7sg - which you will probably need to run several times to get all the words. He was satirising English habits, pretending to give the viewpoints of the people whose nations they had - despite their English "mad dog" habits - conquered or were "managing". Especially so in Queen Victoria's reign, when the TRUE boast was:
"The empire on which the sun never sets".
An astounding phenomenon, far more widespread than the amazing Roman empire. The British Commonwealth of Nations is almost as extensive, but the English monarch no longer RULES most of the member-nations.

💥2: However, the British have long been keen to get out of their cold damp nation (my ancestors got our of Scotland, Wales, England <with a touch of Irish from County Mayo> in roughly 1840 to 1860) and "interfere with" the inhabitants of warmer nations - and when the Muslims were spreading all around the Mediterranean, King John and the Christian Knights were busy waging Holy Crusades against them. The English have, of course, long adored dogs and the nobles developed many of the best known breeds - especially those for hunting (with no wars to fight and no animals to hunt, English nobles DID get very bored!)
I cannot read the Q'ran/Koran, but I'm told that the prophet wrote phrases that have most Muslims despise dogs as "unclean". So what could be more natural than combine their contempt for dogs with their contempt for the English infidels who love dogs? And so they called the English "dogs".

💥3: I have some doubts about the alleged Communist reason for their alleged use of the term. My experience is that dogs would have to be starving to eat a dog. Once it was dead, my dogs weren't interested in the stray cat they killed (Ciwa "herded" the panicked cat to Karl). On the other hand, it is recorded that when the Soviet troops invaded Germany at the end of WW2 they were so short of food that they killed & ate most German civilians' dogs.

💥4: By contrast, Many Brits refer to one another ADMIRINGLY as a "gay dog" (before "gay" was stolen by the homosexual community), a "lucky dog" - and even as a "dirty dog" when jealous of a chap's promiscuous sexual activity.

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King Les The Lofty - first pup in 1950; GSD breeder & trainer as of 1968

Ask them if you want to know, YA isn't research, nor will I do your leg work/research for you.....

Because they think we get upset by it.

Think it is Arabs who tend to use that insult.

(slang, derogatory) A dull, unattractive girl or woman.

(slang) A man (derived from definition 2).
You lucky dog! He's a silly dog.
(slang, derogatory) A coward.
Come back and fight, you dogs!

https://en.wiktionary.org/wiki/dog

http://www.dictionary.com/e/dog/

If someone calls a man a dog, they strongly disapprove of him. If someone, especially a man, calls a woman or girl a dog, they mean that she is very ugly, unattractive, or boring.

https://www.collinsdictionary.com/dictionary/english/dog

https://timesofindia.indiatimes.com/Why-being-called-a-dog-is-a-royal-insult/articleshow/4026729.cms

Possibly the same reason that some women are referred to as "cows".
And why that is...is a mystery, to me anyway.

You may find some interesting info on this issue from this link; https://www.google.co.uk/search?ei=jrKnWpmzHMaSgAbVpaPgDQ&q=what+does+it+mean+to+call+someone+a+dog&oq=what+does+it+mean+to+call+someone+a+dog&gs_l=psy-ab.12..0i7i30k1l8j0.3077.3077.0.4908.1.1.0.0.0.0.67.67.1.1.0....0...1.1.64.psy-ab..0.1.66....0.YJfyAnnPrq0

That's how communists refer to greedy capitalists. It's because capitalists eat each other like a pack of vicious dogs.