There's a massive difference between the two. The Welsh, Scottish and Northern Irish (plus islanders and whatnot) tend to get pretty peeved when you refer to them as English.
I accept that people outside the UK might not care, but I don't think that's necessarily the case. There's also a significant difference between making the mistake in casual conversation and on a formal document.
There's not a massive difference. These are people who speak (for the most part) the same language, have (for the most part) the same religion (Christianity), live a few miles from each others, listen to the same music, watch the same tv shows and read similar books at home and school. Sorry, but for outsiders, you are the same people and for the most part, they'd be right to think so. People are not that different, especially people who have so much in common as the inhabitants of the UK, get over yourself, you're not that special.
I'm sure you don't know and care that southern Peruvians from Arequipa like to be called Arequipaños and wouldn't like being mistaken with Northern Peruvian from the Jungle etc. Thing is they're the same to any outsider.
It's not offensive. Of course it's not. It's just blatantly incorrect. Your taking a group of 4 countries + some islands and referring to them by one of the 4 countries names. It'll confuse people. They now have to think: when he says 'England' does he actually mean England - which would be the obvious meaning - or is he one of those people that has no understanding of geography and is actually referring to a group of several countries? If you refuse to use the correct terminology, even when you are very obviously wrong, you're just confusing people unnecessarily.
This distinction has traditionally never been very important in Germany, so people didn't habitually make the distinction.
That's exactly the point: you have no idea about usage and connotations in German, but still consider yourself an expert who should have the power to prescribe a foreign language.
As an aside: we're also lumping together (dancing) balls and (foot)balls. And lots of other things.
This is the first Germany has been brought up in the conversation. At no point was I discussing use in languages other than English and at no point did I argue that. It seems you argument has fallen apart and you have resorted to changing it.
Because for Swiss people like you, Welsh people like us don't like doing business with Bavarians who call us English. Maybe if you weren't Austrian, you'd understand.
Your examples are completely wrong. The Netherlands is sometimes mistakenly refered to as Holland because this for a time this was the accepted way of referring to the country and its people. It has never been the accepted way to refer to Scottish as English or Welsh as English or Northern Irish as English.
The American comparison would be more suitable if you referred to the whole of the USA as Washington DC.
Interestingly, it used to be pretty common in the US to refer to the Netherlands as Holland--maybe even the norm. Anecdotally, this usage seems far less common today. I have to confess that I didn't know until I looked it up just now that Holland was a region within the Netherlands.
In English--I have no opinion about other languages--it would certainly seem odd to refer to the entire of the UK as "England." For example, you'll get looked at funny if you say you're going to Edinburgh in England next week.
[Edit: My recollection is that it also used to be more common to refer to the "U.K." as Great Britain.]
Because Holland is a region within The Netherlands. Northern Ireland, Scotland, England and Wales are completely separate countries. Holland is not. NI/S/W are not part of England.