How would I know that this thing for example is just a privately issued medal?
Apparently the people that put this new website together don't think it's important to clearly state the difference. I agree that a casual browser using only this website for reference would find it hard to derive the fact that it is not an official legal tender coin; the only such indication is the use of the "X" number system rather than the "KM" number system. But unless you knew to look out for that, you wouldn't know that's what it meant.
The item in the above link is an "aureus magnus" bullion coin; they are listed in the UWC book, under "Germany", presumably because while there is no actual country of issue on the coin, the issuer was a native of Germany. He apparently saw his "coins" as a viable alternative world currency, and they bear coin-like features such as a denomination, so can be classified as a fantasy coin.
Classification of "countries" in UWC is understandably ambiguous; they seem to follow the general rule that if the coin itself clearly states the (alleged) country of issue, then it is listed under that country, but if no such country is named then it is filed under the "usual" Krause name for the country it could reasonably be associated with, either by knowledge of its origin or by similarity in design to real coins from that country.
No idea why United Kingdom coins can only be found if one selects "Great Britain" (what coins would they use in Northern Ireland then?)
"Great Britain" is the name of the entire country, as far as the mainstream Krause catalogues are concerned - even for the pre-1707 coins which technically should be listed under "England". This follows the precedent set by Yeoman, Craig and other early coin cataloguers, most of whom were based in America. UWC is unusual in listing separate entries for "England", "Great Britain" and "United Kingdom", presumably depending on the name actually placed on the fantasy coin.
...or have an option to exclude the "Unusual" pieces catalog
There is a way to do that, but you have to know it's there to use it. Hover your mouse over the
button next to the "catalog initials" field in the "search by criteria" section, and it tells you the catalogues you can choose from, and also mentions that "X" numbers are from UWC. For most countries, all you'd have to do is type "KM" in the catalog initials field and all you'll get back are the legitimate coins.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis