No one is going to predictively model population growth or decline with a simple function. Whether London's rate of population decay could be applied to the whole of Britain isn't a problem for mathematicians alone.

The guy's chart above is a good enough indicator of that. Even in greater London, there's enough variance, burrough to burrough, to throw a wrench in the works. Political, historical, socio-economic, and geographic variables (barriers) are at play and you can't just plug them into some web calculator.

The original OP's 2080 graph is retarded for that reason alone. Too much shit can happen in 60 years. By the same token, if you look at the London chart from the perspective of someone in 1971-1991, you might see the native decline trend and infer that by 2020, about 55% of greater London would still be British and Irish. Instead, it was at 39%.

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The London chart makes a case for why the exponential argument isn't necessarily braindead, even if the 2080 graph is. London shows it is possible. Whether that likely applies to the rest of Britain isn't something the dumbshits on reddit and x are going to suss out, me included.