1 -
No it isn't. Britain is fully attached to Europe. Technically, the UK's borders are made up for the most part by wide rivers indeed, which would lead some to conclude it is an island. This, I am afraid, I must deem cartographicism.
Britain is socio-geographically not an island. Never has been. Corsica is, Iceland is. A thousand tribal and backward places in Europe are or have been. Britain, never. Not since the common Celtic culture, not in the Roman period, not in the age of invasions, not later either. Britain is not an insular country and has not followed a unique historical path. The UK is not a semi-European country. In the sense of Wallerstein or Braudel, the UK has always been part of the core of Europe. The idea of a continent plus a UK is therefore a myth. Contrary to public perception, the UK is amongst the most unlikely candidates to rightfully think of itself as 'different from Europe'.
2 -
I could have started my timeline in 700 BC. And then wrote a hefty post about how Britain's wealth is based on the Italians. I deemed it unfair to point out that Britain has severly lagged behind Italy in GDP per capita for two thousand years. This is why I limited myself to the period of British EU membership. For this time period too, surprisingly, Italy has lead the UK in GDP per capita for longer than the reverse. Not until the early nineties did the UK overtook Italy.
I am quite convinced a majority of British foreign trade is with the EU. Strangely, I can't find a link
3 -
> How many western european countries suffered invasion in the last 1000 odd years?
Lots, including the UK
> How many western european countries suffered invasion in the last 500 odd years?
Lots, including the UK
> How many western european countries suffered invasion in the last 100 odd years?
Lots, excluding the UK.
> How many western european countries suffered invasion in the last 50 odd years?
None.
Which leaves the unfortunate period of 1900-1950 for a British exception. Or 5% of your timeline. Which I would not deem an historical exception.
How successful has Britain been in providing a stable and equitable polity -
> How many western european countries have suffered bloody revolution in the last 350 years?
Lots, including the UK
> How many western european countries have even existed in their current form for the last 175 years?
Lots - but this really depends a good deal on definition.
> How many western european countries suffered the perils of communism?
None
> How many western european countries suffered the perils of facism?
Lots, including Britain.
> How many of the above categories apply to Britain?
All?
How successful has Britain been in promoting sovereirn stability among its neighbours -
> How many western european countries successfully fought off Napoleon?
None.

The Russian winter and the onslaught of the Eastern Despots brought down Napoleon.
> How many western european countries successfully fought off ye olde' Kaiser?
Uhm, that depends who you mean. None in 1866-1870. Lots in 1914-1918.
> How many western european countries successfully fought off Hitler?
None.
The Russian winter and the onslaught of the Eastern Communists brought down Hitler.
> How many western european countries successfully kept communism contained?
All.
> How many nations did all of this and produced none of the mentioned despots?
I end up with 'none, none, none and all'. Which kind of renders it all moot.
The colonial questions are excellent, but really beyond the scope of this post.
I was thinking more about the old British 'Commonwealth first' idea. The, in my eyes, unrealistic belief that the Commonwealth is where Britain's future lies.
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