Prior to 1 January 1949, Indians were British subjects under United Kingdom law. See British nationality law. Between 1 January 1949 and 25 January 1950, Indians remained British subjects without citizenship unless they had already acquired citizenship of the UK & Colonies or another Commonwealth country.
Your terminologies are confused.
You suggested that, if something happened to the British Isles, British culture would be defunct. That is not the case. At the height of the British Empire, there were multiple large territories under nominal British rule where culturally British people of British background were a considerable minority (India, South Africa) or an outright majority (Canada, Australia). British culture would not have gone extinct if the British Isles happened to be destroyed--it would be battered, but the human community of millions of Britons belonging to the same world power would survive.
In the case of the destruction of the British Isles, would there be substantial powers shifts? Sure. Your example of India, where the overwhelming majority of the large population was not of British background, comes to mind. Canada and Australia and maybe even South Africa might well culturally British and even stay part of a post-British empire, but India? It might easily break away. Romulan subject species, like the Kevratans, might do the same after Eisn is scoured.
I think that the novelverse isn't a huge, monolithic entity like some people think it is. There's a subset that has a shared continuity but even within that there's parts that contradict. There's also stand alone novels. If you had enforced your rules on everything fitting into a single continuity then we wouldn't have gotten Diane Duane's Rihannsu or Ford's The Final Reflection.
The novel that you cited as evidence for your thesis about Romulan immobility, Spock's World
, belongs to a particular continuity wherein it's made explicitly clear that the Romulans unlike the Vulcans are an expansionistic, colonizing people. In any case, the Duaneverse continuity has been mostly superseded, partly absorbed, by the current novelverse continuity. (In her later Rihannsu novels, partly written with an eye towards the current continuity, Duane described a much bigger Romulan community that she initially mentioned, with second-generation colonies and "client worlds".) So that doesn't work to support your point.
What does the current novelverse hold as canon? There are multiple large Romulan communities scattered across the Romulan Star Empire, many dating back centuries (Artaleirh), some of these sufficiently large to contend for the position of capital of the Romulan community whether after the brief split (Achernar Prime) or after the destruction of Romulus (Rator III, among others). The current praetor even comes from one of these Romulan worlds (Glintara). What incentive do authors have to revise the novelverse canon so as to eliminate these Romulan-populated worlds? Besides doubling back on a decade's worth of writing, your suggested revision doesn't make sense in light of the way imperialist powers behave and doesn't lend itself to telling interesting stories.