This is also a new one on me. So is the cultural marxism theory a belief in a left-wing elite who seek to destroy Western civilisation through multiculturalism such that they can re-engineer that civilisation along Marxist ideologies?

It'd be a fine theory Rob, Were it not for the fact that "Western civilisation" only exists BECAUSE of immigration. There are no mono cultures left on earth with the exeption of a few untouched tribes.
It's always tickled me how the descendants of immigrants given a few generations forget their own origins and disregard the real reasons for everything ranging from a lack of social housing to unemployment and scapegoat the latest batch of newcomers.
Seriously... Australia First, Britain First, Whichever of the North American parties you choose really have me struggling to contain my laughter and control my bladder, Donald Trump is the obvious example, the guy is descended from a Bulgarian immigrants to America yet the rhetoric he uses would make you think his father was Chief Sitting Bull.
Britain has never stood alone in Europe or the world, either in war or in peace. We are a nation built on trade with others, a nation supplied and defended in times of war by various differing allies from other nations.
For example in the Crimea we fought with the French and the Ottoman Turks against the Russians but in world war I we fought against the Ottoman Turks.
Cornish tin was being sold to Phoenecian traders B.C. and an African woman was living in Yorkshire in the 4th century. Celts landed on the shores of Ireland and may well have been greeted by North Africans who had settled there centuries before the Angles migrated into England.
hmm reminds me of something
Reg: All right, but apart from the sanitation, the medicine, education, wine, public order, irrigation, roads, the fresh-water system, and public health, what have the Romans ever done for us?