So in theory, Cyprus should be on the list and UK not.
Unless you still count rivers as land borders...
Another "good" llustration of the fact is, that when the disaster of hurricane Irma happened (which devastated the entire island 90% of the buildings were damaged, leaving most of them uninhabitable), the Netherlands werent obligated to help, and they werent really entitled to emergency EU funding. Unlike the french side which had the same rights as if it had happened in france, in theory, Sint Maarten were on their own.
oh, and Russia, too!
a1: No man made bodrer
a2: Only visible above sea levels borders (The Oresund bridge would count Denmak out but the Johor–Singapore Causeway would add Singapre to the list and the King Fahd Causeway would add Bahrain to the list. Qatar out but Singapore and Bahrain in. In the future, the proposed Quatar-Bahrain bridge might kick both countries out of the list.)
a3: Bridges and tunnels accepted as border. Kicks UK out of the list.
b1: Overseas territories not taken into account.
b2: Overseas territories counted as separate countries. As an example, this would add Gibraltar to the list.
b3: Overseas territories counted as a part of the mother country. This makes Gibraltar a part of the UK, dropping them from the list.
(Incorporated overseas areas of France an the Netherlands do not affect the list of countries bordering a single country, so we don't have to make a b4.)
... to be continued
c2) De facto indepent countires recognized. This only affects the split of Cyprus. (Interacts with the b1/b2/b3 (like Gibraltar) as the UK owns the Sovereign Base Areas of Akrotiri and Dhekelia, the latter bordering both parts of split-up Cyprus.
d2) disputed islands count as borders. This brings the focus on Hans Island between Greenland and Canda. That defenatly kicks Canda of the list, but it depends on how you treat overseat territories whether is also kicks Denmark out of the list or adds Greenland to it as a seperate state.
e2) Military bases with border control counted as borders. This would add Cuba to the list at Guantanamo bay and probably a lot of other borders around the world as well.
... to be continued.
Have you considered doing another quiz on country pairs with manmade border links (eg Sweden-Denmark, UK-France)? If you already haven't.
Think of it like a person touching your pet dog. They're touching something which belongs to you, and is part of your "family", but they're not touching you.