Let’s unpack the Okinawa fantasy first. Japan has some of the most tight immigration controls in the world. The entire foreign population of Japan is just under 3% so the idea of 800,000 Swedes moving to a Japanese island of 1.3 million and erasing their culture via the establishment of an IKEA on every corner and meatballs for dinner every night is a laughably unrealistic description of immigration.
Multiculturalism doesn’t erase culture, it adds to it and Britain has a long-standing history of diverse ethnic groups. It goes back to my earlier comment of 'they are here because we were there', Britain established itself as the metropole of an empire and so people around the world looked to it as their mother country, they grew up in cultures that had been (forcibly) British-ified. Britain didn’t become “less British” by incorporating other cultures. Culture mixes and adapts.
You say you’re clinging to your identity, but no one’s stopping you from watching Fawlty Towers or reading Wodehouse, Austen has not been outlawed. If your cultural identity is so fragile that the presence of a Turkish barber on the high street sends it into a tailspin, maybe the problem isn’t immigration.
As for the Kalahari Bushman? Bit awkward. Colonised people weren’t upset about too many foreigners moving in, they were upset about being erased by force and that is absolutely not what is happening in Britain. Comparing that to you feeling weird about immigration is, frankly, wild. If you're pining for Austen era values and the days of the Empire, you're not the underdog in the immigration debate.
Ultimately you’re not under attack. You’re just aging in a changing world and blaming brown people for it which, to be fair, is a long-standing British tradition.