First, a disclaimer that will be at the start of the game:
Opinions ascribed to citizens of the Empire in this setting are not those of the author, and some may possibly be considered offensive to a modern audience. These include implied sexism, racism, colonialism, capitalism, jingoism, various forms of double standards, etc. etc. The setting, society and events depicted are not intended to be pleasant or desirable!
Bearing that in mind, here's my version of how race etc. are viewed in the Empire:
One aspect of the modern Empire that would appal its founders is a relatively relaxed attitude to race in the ruling classes. Earlier generations believed in the natural superiority of Anglo-Saxons; eugenics research in the 20th and 21st centuries repeatedly proved that this superiority was in fact an acquired trait, a result of the mixing of the many races which make up the modern Anglo-Saxon heritage. The myth of the superior pure-blooded Englishman or Aryan was thoroughly exploded when statistical analysis proved that inbreeding in the purest families was in fact causing a small but noticeable reduction in cranial size and related loss of intelligence.
This is not to say that indiscriminate promiscuity is encouraged; rather, that the Empire’s ruling classes encourage their children to marry into successful families whose members share their intelligence but are not closely related. Given the Empire’s sheer size and propensity to promote the successful, it was inevitable that occasional members of the so-called lesser races rose to positions of great power, especially in India, China, and other conquered territories, and that occasional Anglo-Saxon woman would find them attractive. Over the generations this has led to some mingling of the races, and marriages that were once considered scandalous are now generally regarded as no more than a little eccentric.
In the lower classes, unfortunately, deprived of the restraints imposed by intelligence, the mingling of races is generally felt to have had more unfortunate effects, with the vices of many nations often manifested in their most lamentable forms. Cheap gin, laudanum, and other drugs are common in the worst slums, as are promiscuity and prostitution, even occasional outbreaks of religious hysteria. The police and public health officers try to eradicate the worst excesses, but are rarely successful for long.
Does this sound like plausible doublethink?