> Its been a downhill slide for the past 200 or so years.
More like 100 IMO, with the death of Queen Victoria. This destabilised all the European monarchies, which within just over a decade led to WW-1 and in so doing sowed the seeds of WW-2.
A thousand years ago, 99.99% of people never left the village / town they were born in. The number of people who undertook foreign travel were very few in number. Wars between nations were very much a local affair, petty squabbles between neighbours really.
Today, international travel is widespread and accessible to a substantial fraction of the population. Information itself travels with such speed as to be practically instantaneous.
Europe has already begun the process of dismantling nation states with ever closer political and monetary union. Other global and regional organisations also act as moderating influences on the worst excesses of any single government.
In a thousand years time, if (as you say) a few dozen idiots don't decide to blow us all up with their delusion of fulfilling a prophecy, the very idea of a nation state may well be an anachronism. With any luck, events like 1950 DA, or others like it, would hopefully teach the world that its better to work together, and all this "us" and "them" primate politics can be finally consigned to history.
Since there are only about 200 countries, it would only take a couple of countries per decade to decide to join up, which seems eminently plausible. The EU has got 25 countries signed up in 50 years after all, with others wanting in.
> Is England Vanishing?
Of course it is, but like the Roman Empire, echoes of it will remain for many millennia. For example, English as a language is sure to endure in one form or another for a long while.